An Introduction to Finite Element Methods Veronika Pillwein RISC ISSAC 2015, 40th International Symposium on Symbolic and Algebraic Computation, Bath, July 6–9, 2015
An Introduction to
Finite Element Methods
Veronika Pillwein
RISC
ISSAC 2015, 40th International Symposium on Symbolic and Algebraic Computation, Bath, July 6–9, 2015
Finite element methods (FEM)
Numerical methods for finding approximatesolutions to partial differential equations(PDEs) on non-trivial domains.
The domain is subdivided into simple geo-metrical objects (triangles, tetrahedra, . . . )and the solution is then approximated bylocally supported, piecewise polynomial basisfunctions.
The task of solving the PDE is reduced to thesolution of a (large scale) linear system.
(Solving the PDE) −→ (Solving Au = f )
Finite element methods (FEM)
Numerical methods for finding approximatesolutions to partial differential equations(PDEs) on non-trivial domains.
The domain is subdivided into simple geo-metrical objects (triangles, tetrahedra, . . . )and the solution is then approximated bylocally supported, piecewise polynomial basisfunctions.
The task of solving the PDE is reduced to thesolution of a (large scale) linear system.
(Solving the PDE) −→ (Solving Au = f )
Finite element methods (FEM)
Numerical methods for finding approximatesolutions to partial differential equations(PDEs) on non-trivial domains.
The domain is subdivided into simple geo-metrical objects (triangles, tetrahedra, . . . )and the solution is then approximated bylocally supported, piecewise polynomial basisfunctions.
The task of solving the PDE is reduced to thesolution of a (large scale) linear system.
(Solving the PDE) −→ (Solving Au = f )
Another non-trivial domain
Another non-trivial domain
Variational/Weak formulation (1D)
I Consider the two-point boundary value problem: given f , findu such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Let v be a sufficiently smooth function with v(0) = 0.
Wemultiply the equation above by v(x) and integrate over Ω.Next we do integration by parts. Now we use the Neumannboundary condition and the essential condition on v .
−∫ 1
0u′′(x)v(x) dx =
∫ 1
0f (x)v(x) dx∫ 1
0u′(x)v ′(x) dx −
[u′(1)v(1)− u′(0)v(0)
]=
∫ 1
0f (x)v(x) dx
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
Variational/Weak formulation (1D)
I Consider the two-point boundary value problem: given f , findu such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Let v be a sufficiently smooth function with v(0) = 0.
Wemultiply the equation above by v(x) and integrate over Ω.Next we do integration by parts. Now we use the Neumannboundary condition and the essential condition on v .
−
∫ 1
0
u′′(x)
v(x) dx
=
∫ 1
0
f (x)
v(x) dx∫ 1
0u′(x)v ′(x) dx −
[u′(1)v(1)− u′(0)v(0)
]=
∫ 1
0f (x)v(x) dx
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
Variational/Weak formulation (1D)
I Consider the two-point boundary value problem: given f , findu such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Let v be a sufficiently smooth function with v(0) = 0. Wemultiply the equation above by v(x)
and integrate over Ω.Next we do integration by parts. Now we use the Neumannboundary condition and the essential condition on v .
−
∫ 1
0
u′′(x)v(x)
dx
=
∫ 1
0
f (x)v(x)
dx∫ 1
0u′(x)v ′(x) dx −
[u′(1)v(1)− u′(0)v(0)
]=
∫ 1
0f (x)v(x) dx
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
Variational/Weak formulation (1D)
I Consider the two-point boundary value problem: given f , findu such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Let v be a sufficiently smooth function with v(0) = 0. Wemultiply the equation above by v(x) and integrate over Ω.
Next we do integration by parts. Now we use the Neumannboundary condition and the essential condition on v .
−∫ 1
0u′′(x)v(x) dx =
∫ 1
0f (x)v(x) dx
∫ 1
0u′(x)v ′(x) dx −
[u′(1)v(1)− u′(0)v(0)
]=
∫ 1
0f (x)v(x) dx
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
Variational/Weak formulation (1D)
I Consider the two-point boundary value problem: given f , findu such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Let v be a sufficiently smooth function with v(0) = 0. Wemultiply the equation above by v(x) and integrate over Ω.Next we do integration by parts.
Now we use the Neumannboundary condition and the essential condition on v .
−∫ 1
0u′′(x)v(x) dx =
∫ 1
0f (x)v(x) dx∫ 1
0u′(x)v ′(x) dx −
[u′(1)v(1)− u′(0)v(0)
]=
∫ 1
0f (x)v(x) dx
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
Variational/Weak formulation (1D)
I Consider the two-point boundary value problem: given f , findu such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Let v be a sufficiently smooth function with v(0) = 0. Wemultiply the equation above by v(x) and integrate over Ω.Next we do integration by parts. Now we use the Neumannboundary condition
and the essential condition on v .
−∫ 1
0u′′(x)v(x) dx =
∫ 1
0f (x)v(x) dx∫ 1
0u′(x)v ′(x) dx −
[u′(1)v(1)− u′(0)v(0)
]=
∫ 1
0f (x)v(x) dx
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
Variational/Weak formulation (1D)
I Consider the two-point boundary value problem: given f , findu such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Let v be a sufficiently smooth function with v(0) = 0. Wemultiply the equation above by v(x) and integrate over Ω.Next we do integration by parts. Now we use the Neumannboundary condition and the essential condition on v .
−∫ 1
0u′′(x)v(x) dx =
∫ 1
0f (x)v(x) dx∫ 1
0u′(x)v ′(x) dx −
[
u′(1)v(1)
− u′(0)v(0)]
=
∫ 1
0f (x)v(x) dx
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
Variational/Weak formulation (1D)
I Consider the two-point boundary value problem: given f , findu such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Let v be a sufficiently smooth function with v(0) = 0. Wemultiply the equation above by v(x) and integrate over Ω.Next we do integration by parts. Now we use the Neumannboundary condition and the essential condition on v .
−∫ 1
0u′′(x)v(x) dx =
∫ 1
0f (x)v(x) dx∫ 1
0u′(x)v ′(x) dx
−[u′(1)v(1)− u′(0)v(0)
]
=
∫ 1
0f (x)v(x) dx
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
Variational/Weak formulation (1D)
I Consider the two-point boundary value problem: given f , findu such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Let v be a sufficiently smooth function with v(0) = 0. Wemultiply the equation above by v(x) and integrate over Ω.Next we do integration by parts. Now we use the Neumannboundary condition and the essential condition on v .
−∫ 1
0u′′(x)v(x) dx =
∫ 1
0f (x)v(x) dx∫ 1
0u′(x)v ′(x) dx
−[u′(1)v(1)− u′(0)v(0)
]
=
∫ 1
0f (x)v(x) dx
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
Variational/Weak formulation (1D)
I Given f ∈ C (0, 1), find u ∈ C 2(0, 1) such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Given f , find u such that
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
for all v sufficiently smooth with v(0) = 0.
Variational/Weak formulation (1D)
I Given f ∈ C (0, 1), find u ∈ C 2(0, 1) such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Given f , find u such that
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
for all v sufficiently smooth with v(0) = 0.
Variational/Weak formulation (1D)
I Given f ∈ C (0, 1), find u ∈ C 2(0, 1) such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Given f , find u such that
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
for all v sufficiently smooth with v(0) = 0.
Galerkin method
I Problem: Given a bilinear form a(·, ·) and a linear form F (·),
find u ∈ V : a(u, v) = F (v) ∀ v ∈ V ,
where V is some infinite dimensional function space.
I Approximate V by finite dimensional subspaces Vh ⊂ V .
I Discrete problem: Given a(·, ·) and F (·),
find uh ∈ Vh : a(uh, vh) = F (vh) ∀ vh ∈ Vh.
I Let Nh = dim Vh and φ1, . . . , φNh be a basis for Vh, then
we can expand
uh(x) =
Nh∑i=1
uiφi (x)
and it is sufficient to consider vh(x) = φj(x) for j = 1, . . . ,Nh.
Galerkin method
I Problem: Given a bilinear form a(·, ·) and a linear form F (·),
find u ∈ V : a(u, v) = F (v) ∀ v ∈ V ,
where V is some infinite dimensional function space.
I Approximate V by finite dimensional subspaces Vh ⊂ V .
I Discrete problem: Given a(·, ·) and F (·),
find uh ∈ Vh : a(uh, vh) = F (vh) ∀ vh ∈ Vh.
I Let Nh = dim Vh and φ1, . . . , φNh be a basis for Vh, then
we can expand
uh(x) =
Nh∑i=1
uiφi (x)
and it is sufficient to consider vh(x) = φj(x) for j = 1, . . . ,Nh.
Galerkin method
I Problem: Given a bilinear form a(·, ·) and a linear form F (·),
find u ∈ V : a(u, v) = F (v) ∀ v ∈ V ,
where V is some infinite dimensional function space.
I Approximate V by finite dimensional subspaces Vh ⊂ V .
I Discrete problem: Given a(·, ·) and F (·),
find uh ∈ Vh : a(uh, vh) = F (vh) ∀ vh ∈ Vh.
I Let Nh = dim Vh and φ1, . . . , φNh be a basis for Vh, then
we can expand
uh(x) =
Nh∑i=1
uiφi (x)
and it is sufficient to consider vh(x) = φj(x) for j = 1, . . . ,Nh.
Galerkin method
I Problem: Given a bilinear form a(·, ·) and a linear form F (·),
find u ∈ V : a(u, v) = F (v) ∀ v ∈ V ,
where V is some infinite dimensional function space.
I Approximate V by finite dimensional subspaces Vh ⊂ V .
I Discrete problem: Given a(·, ·) and F (·),
find uh ∈ Vh : a(uh, vh) = F (vh) ∀ vh ∈ Vh.
I Let Nh = dim Vh and φ1, . . . , φNh be a basis for Vh, then
we can expand
uh(x) =
Nh∑i=1
uiφi (x)
and it is sufficient to consider vh(x) = φj(x) for j = 1, . . . ,Nh.
Galerkin method
I Plug uh(x) =∑Nh
i=1 uiφi (x) and vh(x) = φj(x) into
find uh ∈ Vh : a(uh, vh) = F (vh) ∀ vh ∈ Vh
to obtain
find u ∈ RNh :
Nh∑i=1
uia(φi , φj) = F (φj), ∀ j = 1, . . . ,Nh.
where u = (u1, . . . , uNh).
I LetA = (a(φi , φj)
Nhi ,j=1 and f = (f1, . . . , fNh
),
then we arrive at the linear system
find u ∈ RNh : Au = f .
Galerkin method
I Plug uh(x) =∑Nh
i=1 uiφi (x) and vh(x) = φj(x) into
find uh ∈ Vh : a(uh, vh) = F (vh) ∀ vh ∈ Vh
to obtain
find u ∈ RNh :
Nh∑i=1
uia(φi , φj) = F (φj), ∀ j = 1, . . . ,Nh.
where u = (u1, . . . , uNh).
I LetA = (a(φi , φj)
Nhi ,j=1 and f = (f1, . . . , fNh
),
then we arrive at the linear system
find u ∈ RNh : Au = f .
Variational/Weak formulation (1D)
I Given f ∈ C (0, 1), find u ∈ C 2(0, 1) such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Given f , find u such that
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
for all v sufficiently smooth with v(0) = 0.
I Let
L2(Ω) = f :
∫Ω
f (x)2 dx <∞,
then we require that f ∈ L2(0, 1) and define
V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.
Variational/Weak formulation (1D)
I Given f ∈ C (0, 1), find u ∈ C 2(0, 1) such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Given f , find u such that
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
for all v sufficiently smooth with v(0) = 0.
I Let
L2(Ω) = f :
∫Ω
f (x)2 dx <∞,
then we require that f ∈ L2(0, 1) and define
V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.
Variational/Weak formulation (1D)
I Given f ∈ C (0, 1), find u ∈ C 2(0, 1) such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Given f , find u such that
a(u, v) :=
∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx =: F (v)
for all v sufficiently smooth with v(0) = 0.
I Let
L2(Ω) = f :
∫Ω
f (x)2 dx <∞,
then we require that f ∈ L2(0, 1) and define
V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.
Weak derivative
A function f ∈ L2(a, b) is weakly differentiable, if there existsw ∈ L1
loc(a, b) satisfying∫ b
af (x)v ′(x) dx = −
∫ b
aw(x)v(x) dx
for all v ∈ C∞0 (a, b). If such a w exists, then it is unique a.e.and we write f ′(x) = w(x).
I Weak and strong derivative coincide (in the L2-sense) if f isclassically differentiable.
I Continuous functions that are piecewise differentiable areweakly differentiable.
I Functions with jumps are not weakly differentiable.
Weak derivative
A function f ∈ L2(a, b) is weakly differentiable, if there existsw ∈ L1
loc(a, b) satisfying∫ b
af (x)v ′(x) dx = −
∫ b
aw(x)v(x) dx
for all v ∈ C∞0 (a, b). If such a w exists, then it is unique a.e.and we write f ′(x) = w(x).
I Weak and strong derivative coincide (in the L2-sense) if f isclassically differentiable.
I Continuous functions that are piecewise differentiable areweakly differentiable.
I Functions with jumps are not weakly differentiable.
Weak derivative
A function f ∈ L2(a, b) is weakly differentiable, if there existsw ∈ L1
loc(a, b) satisfying∫ b
af (x)v ′(x) dx = −
∫ b
aw(x)v(x) dx
for all v ∈ C∞0 (a, b). If such a w exists, then it is unique a.e.and we write f ′(x) = w(x).
I Weak and strong derivative coincide (in the L2-sense) if f isclassically differentiable.
I Continuous functions that are piecewise differentiable areweakly differentiable.
I Functions with jumps are not weakly differentiable.
Weak derivative
A function f ∈ L2(a, b) is weakly differentiable, if there existsw ∈ L1
loc(a, b) satisfying∫ b
af (x)v ′(x) dx = −
∫ b
aw(x)v(x) dx
for all v ∈ C∞0 (a, b). If such a w exists, then it is unique a.e.and we write f ′(x) = w(x).
I Weak and strong derivative coincide (in the L2-sense) if f isclassically differentiable.
I Continuous functions that are piecewise differentiable areweakly differentiable.
I Functions with jumps are not weakly differentiable.
Examples
I
g(x) = |x |
- 2 - 1 1 2
0.5
1.0
1.5
2.0
g
- 2 - 1 1 2
- 1.0
- 0.5
0.5
1.0
g ¢
I
g(x) = |(x − 1)(x − 3)(x − 5)|
1 2 3 4 5 6
2
4
6
8
10
g
1 2 3 4 5 6
- 20
- 10
10
20
g ¢
Examples
I
g(x) = |x |
- 2 - 1 1 2
0.5
1.0
1.5
2.0
g
- 2 - 1 1 2
- 1.0
- 0.5
0.5
1.0
g ¢
I
g(x) = |(x − 1)(x − 3)(x − 5)|
1 2 3 4 5 6
2
4
6
8
10
g
1 2 3 4 5 6
- 20
- 10
10
20
g ¢
Model problem in 1D
I Given f ∈ C (0, 1), find u ∈ C 2(0, 1) such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Given f ∈ L2(0, 1), find u ∈ V such that∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx , ∀ v ∈ V ,
where V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.I Find u ∈ RNh :
Au = f ,
where Nh = dim Vh ⊂ V , and the approximate solution isuh(x) =
∑Nhi=1 uiφi (x) for a basis φ1(x), . . . , φNh
(x).
Model problem in 1D
I Given f ∈ C (0, 1), find u ∈ C 2(0, 1) such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Given f ∈ L2(0, 1), find u ∈ V such that∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx , ∀ v ∈ V ,
where V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.
I Find u ∈ RNh :Au = f ,
where Nh = dim Vh ⊂ V , and the approximate solution isuh(x) =
∑Nhi=1 uiφi (x) for a basis φ1(x), . . . , φNh
(x).
Model problem in 1D
I Given f ∈ C (0, 1), find u ∈ C 2(0, 1) such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Given f ∈ L2(0, 1), find u ∈ V such that∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx , ∀ v ∈ V ,
where V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.I Find u ∈ RNh :
Au = f ,
where Nh = dim Vh ⊂ V , and the approximate solution isuh(x) =
∑Nhi=1 uiφi (x) for a basis φ1(x), . . . , φNh
(x).
Model problem in 1D
I Given f ∈ C (0, 1), find u ∈ C 2(0, 1) such that
−u′′(x) = f (x), in Ω = (0, 1)
u(0) = 0, u′(1) = 0.
I Given f ∈ L2(0, 1), find u ∈ V such that∫ 1
0u′(x)v ′(x) dx =
∫ 1
0f (x)v(x) dx , ∀ v ∈ V ,
where V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.I Find u ∈ RNh :
Au = f ,
where Nh = dim Vh ⊂ V , and the approximate solution isuh(x) =
∑Nhi=1 uiφi (x) for a basis φ1(x), . . . , φNh
(x).
Strategies for increasing the accuracy
I initial mesh
I h-version: mesh refinement whilekeeping the polynomial degree fixed
I p-version: increasing thepolynomial degree while keepingthe mesh fixed
I hp-version: combination of bothstrategies
I higher order (i.e., p- and hp-) methods converge faster withrespect to the number of unknowns, but require more effort inthe implementation.
Strategies for increasing the accuracy
I initial mesh
I h-version: mesh refinement whilekeeping the polynomial degree fixed
I p-version: increasing thepolynomial degree while keepingthe mesh fixed
I hp-version: combination of bothstrategies
I higher order (i.e., p- and hp-) methods converge faster withrespect to the number of unknowns, but require more effort inthe implementation.
Strategies for increasing the accuracy
I initial mesh
I h-version: mesh refinement whilekeeping the polynomial degree fixed
I p-version: increasing thepolynomial degree while keepingthe mesh fixed
I hp-version: combination of bothstrategies
I higher order (i.e., p- and hp-) methods converge faster withrespect to the number of unknowns, but require more effort inthe implementation.
Strategies for increasing the accuracy
I initial mesh
I h-version: mesh refinement whilekeeping the polynomial degree fixed
I p-version: increasing thepolynomial degree while keepingthe mesh fixed
I hp-version: combination of bothstrategies
I higher order (i.e., p- and hp-) methods converge faster withrespect to the number of unknowns, but require more effort inthe implementation.
Strategies for increasing the accuracy
I initial mesh
I h-version: mesh refinement whilekeeping the polynomial degree fixed
I p-version: increasing thepolynomial degree while keepingthe mesh fixed
I hp-version: combination of bothstrategies
I higher order (i.e., p- and hp-) methods converge faster withrespect to the number of unknowns, but require more effort inthe implementation.
Conforming finite element bases
I The given domain is subdivided into simple geometric objects(intervals, quadrilaterals, triangles, prisms, tetrahedra, etc.)that are called elements.
I A finite element basis consists of locally supported, piecewisepolynomial basis functions φi (x).
I A finite element basis is conforming if for all i , φi ∈ V , whereV is the function space where the variational problem is posed.
I For 1D we consider next examples for two types of conformingfinite element bases: a nodal basis and a hierarchic basis.
I The basis functions are defined on a reference element andthen mapped to the actual element in the mesh.
Conforming finite element bases
I The given domain is subdivided into simple geometric objects(intervals, quadrilaterals, triangles, prisms, tetrahedra, etc.)that are called elements.
I A finite element basis consists of locally supported, piecewisepolynomial basis functions φi (x).
I A finite element basis is conforming if for all i , φi ∈ V , whereV is the function space where the variational problem is posed.
I For 1D we consider next examples for two types of conformingfinite element bases: a nodal basis and a hierarchic basis.
I The basis functions are defined on a reference element andthen mapped to the actual element in the mesh.
Conforming finite element bases
I The given domain is subdivided into simple geometric objects(intervals, quadrilaterals, triangles, prisms, tetrahedra, etc.)that are called elements.
I A finite element basis consists of locally supported, piecewisepolynomial basis functions φi (x).
I A finite element basis is conforming if for all i , φi ∈ V , whereV is the function space where the variational problem is posed.
I For 1D we consider next examples for two types of conformingfinite element bases: a nodal basis and a hierarchic basis.
I The basis functions are defined on a reference element andthen mapped to the actual element in the mesh.
Conforming finite element bases
I The given domain is subdivided into simple geometric objects(intervals, quadrilaterals, triangles, prisms, tetrahedra, etc.)that are called elements.
I A finite element basis consists of locally supported, piecewisepolynomial basis functions φi (x).
I A finite element basis is conforming if for all i , φi ∈ V , whereV is the function space where the variational problem is posed.
I For 1D we consider next examples for two types of conformingfinite element bases: a nodal basis and a hierarchic basis.
I The basis functions are defined on a reference element andthen mapped to the actual element in the mesh.
Conforming finite element bases
I The given domain is subdivided into simple geometric objects(intervals, quadrilaterals, triangles, prisms, tetrahedra, etc.)that are called elements.
I A finite element basis consists of locally supported, piecewisepolynomial basis functions φi (x).
I A finite element basis is conforming if for all i , φi ∈ V , whereV is the function space where the variational problem is posed.
I For 1D we consider next examples for two types of conformingfinite element bases: a nodal basis and a hierarchic basis.
I The basis functions are defined on a reference element andthen mapped to the actual element in the mesh.
Setting for the 1D model problem
I Given f ∈ L2(0, 1), find u ∈ V such that∫ 1
0u′(x)v ′(x) dx︸ ︷︷ ︸
=a(u,v)
=
∫ 1
0f (x)v(x) dx︸ ︷︷ ︸
=F (v)
, ∀ v ∈ V ,
where V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.
I Regularity requirements for the basis functions: weaklydifferentiable and square integrable on (0, 1).
We will definecontinuous, piecewise polynomial basis functions.
I For sake of simplicity we use an equidistant subdivision of(0, 1) with the same maximal polynomial degree p on eachelement.
Note that the construction we present allows to varythe polynomial degree on the elements.
Setting for the 1D model problem
I Given f ∈ L2(0, 1), find u ∈ V such that∫ 1
0u′(x)v ′(x) dx︸ ︷︷ ︸
=a(u,v)
=
∫ 1
0f (x)v(x) dx︸ ︷︷ ︸
=F (v)
, ∀ v ∈ V ,
where V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.I Regularity requirements for the basis functions: weakly
differentiable and square integrable on (0, 1).
We will definecontinuous, piecewise polynomial basis functions.
I For sake of simplicity we use an equidistant subdivision of(0, 1) with the same maximal polynomial degree p on eachelement.
Note that the construction we present allows to varythe polynomial degree on the elements.
Setting for the 1D model problem
I Given f ∈ L2(0, 1), find u ∈ V such that∫ 1
0u′(x)v ′(x) dx︸ ︷︷ ︸
=a(u,v)
=
∫ 1
0f (x)v(x) dx︸ ︷︷ ︸
=F (v)
, ∀ v ∈ V ,
where V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.I Regularity requirements for the basis functions: weakly
differentiable and square integrable on (0, 1). We will definecontinuous, piecewise polynomial basis functions.
I For sake of simplicity we use an equidistant subdivision of(0, 1) with the same maximal polynomial degree p on eachelement.
Note that the construction we present allows to varythe polynomial degree on the elements.
Setting for the 1D model problem
I Given f ∈ L2(0, 1), find u ∈ V such that∫ 1
0u′(x)v ′(x) dx︸ ︷︷ ︸
=a(u,v)
=
∫ 1
0f (x)v(x) dx︸ ︷︷ ︸
=F (v)
, ∀ v ∈ V ,
where V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.I Regularity requirements for the basis functions: weakly
differentiable and square integrable on (0, 1). We will definecontinuous, piecewise polynomial basis functions.
I For sake of simplicity we use an equidistant subdivision of(0, 1) with the same maximal polynomial degree p on eachelement.
Note that the construction we present allows to varythe polynomial degree on the elements.
Setting for the 1D model problem
I Given f ∈ L2(0, 1), find u ∈ V such that∫ 1
0u′(x)v ′(x) dx︸ ︷︷ ︸
=a(u,v)
=
∫ 1
0f (x)v(x) dx︸ ︷︷ ︸
=F (v)
, ∀ v ∈ V ,
where V = v ∈ L2(0, 1) : a(v , v) <∞ and v(0) = 0.I Regularity requirements for the basis functions: weakly
differentiable and square integrable on (0, 1). We will definecontinuous, piecewise polynomial basis functions.
I For sake of simplicity we use an equidistant subdivision of(0, 1) with the same maximal polynomial degree p on eachelement. Note that the construction we present allows to varythe polynomial degree on the elements.
High order nodal finite element basis
I reference element I = [−1, 1], maximal polynomial degree p
I Lagrange basis polynomials:
`j(x) =∏i 6=j
x − xixj − xi
,
where x1, . . . , xp+1 is a
n equidistant
subdivision of I.
I Example p = 3:
-1.0 -0.5 0.5 1.0
-0.2
0.2
0.4
0.6
0.8
1.0
High order nodal finite element basis
I reference element I = [−1, 1], maximal polynomial degree p
I Lagrange basis polynomials:
`j(x) =∏i 6=j
x − xixj − xi
,
where x1, . . . , xp+1 is a
n equidistant
subdivision of I.
I Example p = 3:
-1.0 -0.5 0.5 1.0
-0.2
0.2
0.4
0.6
0.8
1.0
High order nodal finite element basis
I reference element I = [−1, 1], maximal polynomial degree p
I Lagrange basis polynomials:
`j(x) =∏i 6=j
x − xixj − xi
,
where x1, . . . , xp+1 is an equidistant subdivision of I.
I Example p = 3:
-1.0 -0.5 0.5 1.0
-0.2
0.2
0.4
0.6
0.8
1.0
High order nodal finite element basis
I reference element I = [−1, 1], maximal polynomial degree p
I Lagrange basis polynomials:
`j(x) =∏i 6=j
x − xixj − xi
,
where x1, . . . , xp+1 is an equidistant subdivision of I.
I Example p = 3:
-1.0 -0.5 0.5 1.0
-0.2
0.2
0.4
0.6
0.8
1.0
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x)
, `(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x)
, `(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x)
, `(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)
→ local basis functionsφ1(x), φ2(x), φ3(x), φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x),
`(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x),
φ2(x), φ3(x), φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x),
`(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x),
φ3(x), φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x),
`(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x),
φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x),
φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x), φ5(x),
φ6(x), φ7(x), φ8(x)→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x), φ5(x),φ6(x),
φ7(x), φ8(x)→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x), φ5(x),φ6(x), φ7(x),
φ8(x)→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
`1(x), `2(x), `3(x)
`(1)1 (x), `
(1)2 (x), `
(1)3 (x), `
(2)1 (x), `
(2)2 (x),
`(2)3 (x), `
(3)1 (x), `
(3)2 (x), `
(3)3 (x), `
(4)1 (x),
`(4)2 (x), `
(4)3 (x)→ local basis functions
φ1(x), φ2(x), φ3(x), φ4(x), φ5(x),φ6(x), φ7(x), φ8(x)
→ global basis functions
The system matrix
I A = (a(φi , φj)Nhi ,j=1 with a(u, v) =
∫ 10 u′(x)v ′(x) dx .
A =
JI JI 0 0 0 0 0 0JI JI JI JI 0 0 0 0
0 JI JI JI 0 0 0 00 JI JI JI JI JI 0 00 0 0 JI JI JI 0 00 0 0 JI JI JI JI JI0 0 0 0 0 JI JI JI0 0 0 0 0 JI JI JI
14
12
34
1
12
1
The system matrix
I A = (a(φi , φj)Nhi ,j=1 with a(u, v) =
∫ 10 u′(x)v ′(x) dx .
A =
JI JI 0 0 0 0 0 0JI JI JI JI 0 0 0 0
0 JI JI JI 0 0 0 00 JI JI JI JI JI 0 00 0 0 JI JI JI 0 00 0 0 JI JI JI JI JI0 0 0 0 0 JI JI JI0 0 0 0 0 JI JI JI
14
12
34
1
12
1
The system matrix
I A = (a(φi , φj)Nhi ,j=1 with a(u, v) =
∫ 10 u′(x)v ′(x) dx .
A =
JI JI 0 0 0 0 0 0JI JI JI JI 0 0 0 0
0 JI JI JI 0 0 0 00 JI JI JI JI JI 0 00 0 0 JI JI JI 0 00 0 0 JI JI JI JI JI0 0 0 0 0 JI JI JI0 0 0 0 0 JI JI JI
14
12
34
1
12
1
High order hierarchical finite element basisI reference element I = [−1, 1], maximal polynomial degree p
I vertex based basis functions: 1 in the defining vertex and 0 inall other vertices,
ϕV ,0(x) =1− x
2, ϕV ,1(x) =
1 + x
2I cell based basis functions: vanish at the boundary of the
element, polynomial inside:
ϕC ,i (x) = Li (x) :=
∫ x
−1Pi−1(x) dx , i ≥ 2,
where Pn(x) is the nth Legendre polynomial. The polynomialsLn(x) are called integrated Legendre polynomials.
I Legendre polynomials are orthogonal w.r.t. the L2(−1, 1)inner product, i.e.,∫ 1
−1Pi (x)Pj(x) dx = 0, if i 6= j .
High order hierarchical finite element basisI reference element I = [−1, 1], maximal polynomial degree pI vertex based basis functions: 1 in the defining vertex and 0 in
all other vertices,
ϕV ,0(x) =1− x
2, ϕV ,1(x) =
1 + x
2
I cell based basis functions: vanish at the boundary of theelement, polynomial inside:
ϕC ,i (x) = Li (x) :=
∫ x
−1Pi−1(x) dx , i ≥ 2,
where Pn(x) is the nth Legendre polynomial. The polynomialsLn(x) are called integrated Legendre polynomials.
I Legendre polynomials are orthogonal w.r.t. the L2(−1, 1)inner product, i.e.,∫ 1
−1Pi (x)Pj(x) dx = 0, if i 6= j .
High order hierarchical finite element basisI reference element I = [−1, 1], maximal polynomial degree pI vertex based basis functions: 1 in the defining vertex and 0 in
all other vertices,
ϕV ,0(x) =1− x
2, ϕV ,1(x) =
1 + x
2I cell based basis functions: vanish at the boundary of the
element, polynomial inside:
ϕC ,i (x) = Li (x) :=
∫ x
−1Pi−1(x) dx , i ≥ 2,
where Pn(x) is the nth Legendre polynomial. The polynomialsLn(x) are called integrated Legendre polynomials.
I Legendre polynomials are orthogonal w.r.t. the L2(−1, 1)inner product, i.e.,∫ 1
−1Pi (x)Pj(x) dx = 0, if i 6= j .
Integrated Legendre polynomialsI For n ≥ 2 with L0(x) = −1 and L1(x) = x we have
Ln(x) =2n − 3
nxLn−1(x)− n − 3
nLn−2(x).
I Ln(±1) = 0I Orthogonality relation∫ 1
−1L′i (x)L′j(x) dx = 0, if i 6= j
I Sparse w.r.t. the L2 inner product∫ 1
−1Li (x)Lj(x) dx = 0, if |i − j | 6= 0, 2
- 1.0 - 0.5 0.5 1.0
- 0.4
- 0.2
0.2
Integrated Legendre polynomialsI For n ≥ 2 with L0(x) = −1 and L1(x) = x we have
Ln(x) =2n − 3
nxLn−1(x)− n − 3
nLn−2(x).
I Ln(±1) = 0
I Orthogonality relation∫ 1
−1L′i (x)L′j(x) dx = 0, if i 6= j
I Sparse w.r.t. the L2 inner product∫ 1
−1Li (x)Lj(x) dx = 0, if |i − j | 6= 0, 2
- 1.0 - 0.5 0.5 1.0
- 0.4
- 0.2
0.2
Integrated Legendre polynomialsI For n ≥ 2 with L0(x) = −1 and L1(x) = x we have
Ln(x) =2n − 3
nxLn−1(x)− n − 3
nLn−2(x).
I Ln(±1) = 0I Orthogonality relation∫ 1
−1L′i (x)L′j(x) dx = 0, if i 6= j
I Sparse w.r.t. the L2 inner product∫ 1
−1Li (x)Lj(x) dx = 0, if |i − j | 6= 0, 2
- 1.0 - 0.5 0.5 1.0
- 0.4
- 0.2
0.2
Integrated Legendre polynomialsI For n ≥ 2 with L0(x) = −1 and L1(x) = x we have
Ln(x) =2n − 3
nxLn−1(x)− n − 3
nLn−2(x).
I Ln(±1) = 0I Orthogonality relation∫ 1
−1L′i (x)L′j(x) dx = 0, if i 6= j
I Sparse w.r.t. the L2 inner product∫ 1
−1Li (x)Lj(x) dx = 0, if |i − j | 6= 0, 2
- 1.0 - 0.5 0.5 1.0
- 0.4
- 0.2
0.2
Local and global basis (p = 2, 4 elements)
14
12
34
1
12
1
-1 -
12
12
1
12
1
ϕV ,0(x), ϕV ,1(x), ϕC ,2(x)
global basis:φV2(x), φV3(x), φV4(x), φV5(x)︸ ︷︷ ︸
=ΦV
,
φC1,2(x), φC2,2(x), φC3,2(x), φC4,2(x)︸ ︷︷ ︸=ΦC
The system matrix
I A = (a(φi , φj)Nhi ,j=1 =
(AVV AVC
ACV ACC
)with a(u, v)=
1∫0
u′v ′dx .
A =
JI JI 0 0 JI JI 0 0JI JI JI 0 0 JI JI 0
0 JI JI JI 0 0 JI JI0 0 JI JI 0 0 0 JIJI 0 0 0 JI 0 0 0JI JI 0 0 0 JI 0 0
0 JI JI 0 0 0 JI 00 0 JI JI 0 0 0 JI
14
12
34
1
12
1
The system matrix
I A = (a(φi , φj)Nhi ,j=1 =
(AVV AVC
ACV ACC
)with a(u, v)=
1∫0
u′v ′dx .
A =
JI JI 0 0 JI JI 0 0JI JI JI 0 0 JI JI 0
0 JI JI JI 0 0 JI JI0 0 JI JI 0 0 0 JI
JI 0 0 0 JI 0 0 0JI JI 0 0 0 JI 0 0
0 JI JI 0 0 0 JI 00 0 JI JI 0 0 0 JI
14
12
34
1
12
1
Example
Find u : − u′′(x) = 18 (2x − 1)(4x − 3) sin(πx), in (0, 1)
u(0) = 0, u′(1) = 0.
Example
Find u : − u′′(x) = 18 (2x − 1)(4x − 3) sin(πx), in (0, 1)
u(0) = 0, u′(1) = 0.
0.0 0.2 0.4 0.6 0.8 1.0
0.002
0.004
0.006
0.008
exact solution
Example
Find u : − u′′(x) = 18 (2x − 1)(4x − 3) sin(πx), in (0, 1)
u(0) = 0, u′(1) = 0.
0.0 0.2 0.4 0.6 0.8 1.0
0.002
0.004
0.006
0.008
exact solution, approximation for p = 2
Example
Find u : − u′′(x) = 18 (2x − 1)(4x − 3) sin(πx), in (0, 1)
u(0) = 0, u′(1) = 0.
0.0 0.2 0.4 0.6 0.8 1.0
0.002
0.004
0.006
0.008
exact solution, approximation for p = 3
Example
Find u : − u′′(x) = 18 (2x − 1)(4x − 3) sin(πx), in (0, 1)
u(0) = 0, u′(1) = 0.
0.0 0.2 0.4 0.6 0.8 1.0
0.002
0.004
0.006
0.008
exact solution, approximation for p = 4
Example
Find u : − u′′(x) = 18 (2x − 1)(4x − 3) sin(πx), in (0, 1)
u(0) = 0, u′(1) = 0.
0.0 0.2 0.4 0.6 0.8 1.0
0.002
0.004
0.006
0.008
exact solution, approximation for p = 5
Example
Find u : − u′′(x) = 18 (2x − 1)(4x − 3) sin(πx), in (0, 1)
u(0) = 0, u′(1) = 0.
0.0 0.2 0.4 0.6 0.8 1.0
0.002
0.004
0.006
0.008
exact solution, approximation for p = 6
Example: the system matrix for p = 6
in the hierarchic basis:
1 0 0 0 0 0
0 43 0 0 0 0
0 0 45 0 0 0
0 0 0 47 0 0
0 0 0 0 49 0
0 0 0 0 0 411
I sparse elementmatrix
I add newpolynomials whenincreasing thepolynomial degree
I recurrence for fastevaluation
In the nodal basis:
40. −48. 38. −24. 10. −2.2−48. 80. −76. 50. −24. 5.938. −76. 95. −76. 38. −9.8−24. 50. −76. 80. −48. 11.10. −24. 38. −48. 40. −14.−2.2 5.9 −9.8 11. −14. 8.6
I full element matrix
I recompute whole basis whenincreasing the polynomial degree
Example: the system matrix for p = 6
in the hierarchic basis:
1 0 0 0 0 0
0 43 0 0 0 0
0 0 45 0 0 0
0 0 0 47 0 0
0 0 0 0 49 0
0 0 0 0 0 411
I sparse element
matrix
I add newpolynomials whenincreasing thepolynomial degree
I recurrence for fastevaluation
In the nodal basis:
40. −48. 38. −24. 10. −2.2−48. 80. −76. 50. −24. 5.938. −76. 95. −76. 38. −9.8−24. 50. −76. 80. −48. 11.10. −24. 38. −48. 40. −14.−2.2 5.9 −9.8 11. −14. 8.6
I full element matrix
I recompute whole basis whenincreasing the polynomial degree
Example: the system matrix for p = 6
in the hierarchic basis:
1 0 0 0 0 0
0 43 0 0 0 0
0 0 45 0 0 0
0 0 0 47 0 0
0 0 0 0 49 0
0 0 0 0 0 411
I sparse element
matrix
I add newpolynomials whenincreasing thepolynomial degree
I recurrence for fastevaluation
In the nodal basis:
40. −48. 38. −24. 10. −2.2−48. 80. −76. 50. −24. 5.938. −76. 95. −76. 38. −9.8−24. 50. −76. 80. −48. 11.10. −24. 38. −48. 40. −14.−2.2 5.9 −9.8 11. −14. 8.6
I full element matrix
I recompute whole basis whenincreasing the polynomial degree
Example: the system matrix for p = 6
in the hierarchic basis:
1 0 0 0 0 0
0 43 0 0 0 0
0 0 45 0 0 0
0 0 0 47 0 0
0 0 0 0 49 0
0 0 0 0 0 411
I sparse element
matrix
I add newpolynomials whenincreasing thepolynomial degree
I recurrence for fastevaluation
In the nodal basis:
40. −48. 38. −24. 10. −2.2−48. 80. −76. 50. −24. 5.938. −76. 95. −76. 38. −9.8−24. 50. −76. 80. −48. 11.10. −24. 38. −48. 40. −14.−2.2 5.9 −9.8 11. −14. 8.6
I full element matrix
I recompute whole basis whenincreasing the polynomial degree
Variational/Weak formulation (2D)
I Model problem: given f : R2 → R, find u : R2 → R s.t.
−∆u(x) = f (x), in Ω
u(x) = 0, on ∂Ω,
where ∆g(x) =∑d
i=1∂2g∂x2
i(x) denotes the Laplace operator.
I Let v be a sufficiently smooth function with v |∂Ω= 0. Wemultiply the equation above by v(x) and integrate over Ω.
Next we apply Gauß’ theorem and use the essential conditionon v .
−∫
Ω∆u(x)v(x) dx =
∫Ω
f (x)v(x) dx
∫Ω∇u(x)∇v(x) dx −
∫∂Ω
∂u
∂n(x)v(x) dx =
∫Ω
f (x)v(x) dx
a(u, v) :=
∫Ω∇u(x)∇v(x) dx =
∫Ω
f (x)v(x) dx =: F (v)
Variational/Weak formulation (2D)
I Model problem: given f : R2 → R, find u : R2 → R s.t.
−∆u(x) = f (x), in Ω
u(x) = 0, on ∂Ω,
where ∆g(x) =∑d
i=1∂2g∂x2
i(x) denotes the Laplace operator.
I Let v be a sufficiently smooth function with v |∂Ω= 0. Wemultiply the equation above by v(x) and integrate over Ω.
Next we apply Gauß’ theorem and use the essential conditionon v .
−∫
Ω∆u(x)v(x) dx =
∫Ω
f (x)v(x) dx
∫Ω∇u(x)∇v(x) dx −
∫∂Ω
∂u
∂n(x)v(x) dx =
∫Ω
f (x)v(x) dx
a(u, v) :=
∫Ω∇u(x)∇v(x) dx =
∫Ω
f (x)v(x) dx =: F (v)
Variational/Weak formulation (2D)
I Model problem: given f : R2 → R, find u : R2 → R s.t.
−∆u(x) = f (x), in Ω
u(x) = 0, on ∂Ω,
where ∆g(x) =∑d
i=1∂2g∂x2
i(x) denotes the Laplace operator.
I Let v be a sufficiently smooth function with v |∂Ω= 0. Wemultiply the equation above by v(x) and integrate over Ω.Next we apply Gauß’ theorem
and use the essential conditionon v .
−∫
Ω∆u(x)v(x) dx =
∫Ω
f (x)v(x) dx∫Ω∇u(x)∇v(x) dx −
∫∂Ω
∂u
∂n(x)v(x) dx =
∫Ω
f (x)v(x) dx
a(u, v) :=
∫Ω∇u(x)∇v(x) dx =
∫Ω
f (x)v(x) dx =: F (v)
Variational/Weak formulation (2D)
I Model problem: given f : R2 → R, find u : R2 → R s.t.
−∆u(x) = f (x), in Ω
u(x) = 0, on ∂Ω,
where ∆g(x) =∑d
i=1∂2g∂x2
i(x) denotes the Laplace operator.
I Let v be a sufficiently smooth function with v |∂Ω= 0. Wemultiply the equation above by v(x) and integrate over Ω.Next we apply Gauß’ theorem and use the essential conditionon v .
−∫
Ω∆u(x)v(x) dx =
∫Ω
f (x)v(x) dx∫Ω∇u(x)∇v(x) dx −
∫∂Ω
∂u
∂n(x)v(x) dx =
∫Ω
f (x)v(x) dx
a(u, v) :=
∫Ω∇u(x)∇v(x) dx =
∫Ω
f (x)v(x) dx =: F (v)
Variational/Weak formulation (2D)
I Model problem: given f : R2 → R, find u : R2 → R s.t.
−∆u(x) = f (x), in Ω
u(x) = 0, on ∂Ω,
where ∆g(x) =∑d
i=1∂2g∂x2
i(x) denotes the Laplace operator.
I Let v be a sufficiently smooth function with v |∂Ω= 0. Wemultiply the equation above by v(x) and integrate over Ω.Next we apply Gauß’ theorem and use the essential conditionon v .
−∫
Ω∆u(x)v(x) dx =
∫Ω
f (x)v(x) dx∫Ω∇u(x)∇v(x) dx
−∫∂Ω
∂u
∂n(x)v(x) dx
=
∫Ω
f (x)v(x) dx
a(u, v) :=
∫Ω∇u(x)∇v(x) dx =
∫Ω
f (x)v(x) dx =: F (v)
Model problem in 2D
I Given f ∈ C (Ω), find u ∈ C 2(Ω) such that
−∆u(x) = f (x), in Ω
u(x) = 0, on ∂Ω,
I Given f ∈ L2(Ω), find u ∈ V such that∫Ω∇u(x)∇v(x) dx =
∫Ω
f (x)v(x) dx , ∀ v ∈ V ,
where V = v ∈ L2(Ω): a(v , v) <∞ and v |∂Ω= 0.I Find u ∈ RNh :
Au = f ,
where Nh = dim Vh ⊂ V , and the approximate solution isuh(x) =
∑Nhi=1 uiφi (x) for a basis φ1(x), . . . , φNh
(x).
Hierarchic high order finite element basis in 2D
I Vertex based basis functions
1 at the defining vertex, 0 on allother vertices, linear in between
I Edge based basis functions
polynomial on the defining edge,vanishing on all other edges, polynomial in between
I Cell based basis functions
supported only on the definingelement, i.e., vanish on the boundary, polynomial on theinterior of the element
Hierarchic high order finite element basis in 2D
I Vertex based basis functions 1 at the defining vertex, 0 on allother vertices, linear in between
I Edge based basis functions
polynomial on the defining edge,vanishing on all other edges, polynomial in between
I Cell based basis functions
supported only on the definingelement, i.e., vanish on the boundary, polynomial on theinterior of the element
Hierarchic high order finite element basis in 2D
I Vertex based basis functions 1 at the defining vertex, 0 on allother vertices, linear in between
I Edge based basis functions polynomial on the defining edge,vanishing on all other edges, polynomial in between
I Cell based basis functions
supported only on the definingelement, i.e., vanish on the boundary, polynomial on theinterior of the element
Hierarchic high order finite element basis in 2D
I Vertex based basis functions 1 at the defining vertex, 0 on allother vertices, linear in between
I Edge based basis functions polynomial on the defining edge,vanishing on all other edges, polynomial in between
I Cell based basis functions supported only on the definingelement, i.e., vanish on the boundary, polynomial on theinterior of the element
High order finite element basis on quadrilateralsI reference element Q = [−1, 1]2
I tensor product of 1D basis functions
I Vertex based basis functions:
ϕV1(x , y) = ϕV ,0(x)ϕV ,0(y)
= 14 (1− x)(1− y)
V1 V2
V3V4
e1
e2
e3
e4
I Edge based basis functions: ϕe1,i (x , y) = Li (x)ϕV ,0(y), fori = 2, . . . , p
I Cell based basis functions: ϕC ,i ,j(x , y) = Li (x)Lj(y) fori , j = 2, . . . , p
I Local vector of basis functions:
(ϕV1 , ϕV2 , ϕV3 , ϕV4︸ ︷︷ ︸ϕV
, ϕe1,2, . . . , ϕe4,p︸ ︷︷ ︸ϕE
, ϕC ,2,2, . . . , ϕC ,p,p︸ ︷︷ ︸ϕC
)
High order finite element basis on quadrilateralsI reference element Q = [−1, 1]2
I tensor product of 1D basis functions
I Vertex based basis functions:
ϕV1(x , y) = ϕV ,0(x)ϕV ,0(y)
= 14 (1− x)(1− y)
V1 V2
V3V4
e1
e2
e3
e4
I Edge based basis functions: ϕe1,i (x , y) = Li (x)ϕV ,0(y), fori = 2, . . . , p
I Cell based basis functions: ϕC ,i ,j(x , y) = Li (x)Lj(y) fori , j = 2, . . . , p
I Local vector of basis functions:
(ϕV1 , ϕV2 , ϕV3 , ϕV4︸ ︷︷ ︸ϕV
, ϕe1,2, . . . , ϕe4,p︸ ︷︷ ︸ϕE
, ϕC ,2,2, . . . , ϕC ,p,p︸ ︷︷ ︸ϕC
)
High order finite element basis on quadrilateralsI reference element Q = [−1, 1]2
I tensor product of 1D basis functions
I Vertex based basis functions:
ϕV1(x , y) = ϕV ,0(x)ϕV ,0(y)
= 14 (1− x)(1− y)
V1 V2
V3V4
e1
e2
e3
e4
I Edge based basis functions: ϕe1,i (x , y) = Li (x)ϕV ,0(y), fori = 2, . . . , p
I Cell based basis functions: ϕC ,i ,j(x , y) = Li (x)Lj(y) fori , j = 2, . . . , p
I Local vector of basis functions:
(ϕV1 , ϕV2 , ϕV3 , ϕV4︸ ︷︷ ︸ϕV
, ϕe1,2, . . . , ϕe4,p︸ ︷︷ ︸ϕE
, ϕC ,2,2, . . . , ϕC ,p,p︸ ︷︷ ︸ϕC
)
High order finite element basis on quadrilateralsI reference element Q = [−1, 1]2
I tensor product of 1D basis functions
I Vertex based basis functions:
ϕV1(x , y) = ϕV ,0(x)ϕV ,0(y)
= 14 (1− x)(1− y)
-1.0
-0.5
0.0
0.5
1.0-1.0
-0.5
0.0
0.5
1.0
0.0
0.5
1.0
I Edge based basis functions: ϕe1,i (x , y) = Li (x)ϕV ,0(y), fori = 2, . . . , p
I Cell based basis functions: ϕC ,i ,j(x , y) = Li (x)Lj(y) fori , j = 2, . . . , p
I Local vector of basis functions:
(ϕV1 , ϕV2 , ϕV3 , ϕV4︸ ︷︷ ︸ϕV
, ϕe1,2, . . . , ϕe4,p︸ ︷︷ ︸ϕE
, ϕC ,2,2, . . . , ϕC ,p,p︸ ︷︷ ︸ϕC
)
High order finite element basis on quadrilateralsI reference element Q = [−1, 1]2
I tensor product of 1D basis functions
I Vertex based basis functions:
ϕV1(x , y) = ϕV ,0(x)ϕV ,0(y)
= 14 (1− x)(1− y)
V1 V2
V3V4
e1
e2
e3
e4
I Edge based basis functions: ϕe1,i (x , y) = Li (x)ϕV ,0(y), fori = 2, . . . , p
I Cell based basis functions: ϕC ,i ,j(x , y) = Li (x)Lj(y) fori , j = 2, . . . , p
I Local vector of basis functions:
(ϕV1 , ϕV2 , ϕV3 , ϕV4︸ ︷︷ ︸ϕV
, ϕe1,2, . . . , ϕe4,p︸ ︷︷ ︸ϕE
, ϕC ,2,2, . . . , ϕC ,p,p︸ ︷︷ ︸ϕC
)
High order finite element basis on quadrilateralsI reference element Q = [−1, 1]2
I tensor product of 1D basis functions
I Vertex based basis functions:
ϕV1(x , y) = ϕV ,0(x)ϕV ,0(y)
= 14 (1− x)(1− y)
-1.0
-0.5
0.0
0.5
1.0-1.0
-0.5
0.0
0.5
1.0
-0.2
-0.1
0.0
0.1
0.2
I Edge based basis functions: ϕe1,i (x , y) = Li (x)ϕV ,0(y), fori = 2, . . . , p
I Cell based basis functions: ϕC ,i ,j(x , y) = Li (x)Lj(y) fori , j = 2, . . . , p
I Local vector of basis functions:
(ϕV1 , ϕV2 , ϕV3 , ϕV4︸ ︷︷ ︸ϕV
, ϕe1,2, . . . , ϕe4,p︸ ︷︷ ︸ϕE
, ϕC ,2,2, . . . , ϕC ,p,p︸ ︷︷ ︸ϕC
)
High order finite element basis on quadrilateralsI reference element Q = [−1, 1]2
I tensor product of 1D basis functions
I Vertex based basis functions:
ϕV1(x , y) = ϕV ,0(x)ϕV ,0(y)
= 14 (1− x)(1− y)
V1 V2
V3V4
e1
e2
e3
e4
I Edge based basis functions: ϕe1,i (x , y) = Li (x)ϕV ,0(y), fori = 2, . . . , p
I Cell based basis functions: ϕC ,i ,j(x , y) = Li (x)Lj(y) fori , j = 2, . . . , p
I Local vector of basis functions:
(ϕV1 , ϕV2 , ϕV3 , ϕV4︸ ︷︷ ︸ϕV
, ϕe1,2, . . . , ϕe4,p︸ ︷︷ ︸ϕE
, ϕC ,2,2, . . . , ϕC ,p,p︸ ︷︷ ︸ϕC
)
High order finite element basis on quadrilateralsI reference element Q = [−1, 1]2
I tensor product of 1D basis functions
I Vertex based basis functions:
ϕV1(x , y) = ϕV ,0(x)ϕV ,0(y)
= 14 (1− x)(1− y)
-1.0
-0.5
0.0
0.5
1.0-1.0
-0.5
0.0
0.5
1.0
-0.10
-0.05
0.00
0.05
0.10
I Edge based basis functions: ϕe1,i (x , y) = Li (x)ϕV ,0(y), fori = 2, . . . , p
I Cell based basis functions: ϕC ,i ,j(x , y) = Li (x)Lj(y) fori , j = 2, . . . , p
I Local vector of basis functions:
(ϕV1 , ϕV2 , ϕV3 , ϕV4︸ ︷︷ ︸ϕV
, ϕe1,2, . . . , ϕe4,p︸ ︷︷ ︸ϕE
, ϕC ,2,2, . . . , ϕC ,p,p︸ ︷︷ ︸ϕC
)
High order finite element basis on quadrilateralsI reference element Q = [−1, 1]2
I tensor product of 1D basis functions
I Vertex based basis functions:
ϕV1(x , y) = ϕV ,0(x)ϕV ,0(y)
= 14 (1− x)(1− y)
V1 V2
V3V4
e1
e2
e3
e4
I Edge based basis functions: ϕe1,i (x , y) = Li (x)ϕV ,0(y), fori = 2, . . . , p
I Cell based basis functions: ϕC ,i ,j(x , y) = Li (x)Lj(y) fori , j = 2, . . . , p
I Local vector of basis functions:
(ϕV1 , ϕV2 , ϕV3 , ϕV4︸ ︷︷ ︸ϕV
, ϕe1,2, . . . , ϕe4,p︸ ︷︷ ︸ϕE
, ϕC ,2,2, . . . , ϕC ,p,p︸ ︷︷ ︸ϕC
)
Global vector of basis functions
I Collect globally vertex, edge and cellbased basis functions (ΦV ,ΦE ,ΦC )
I Vertex based basis functions aresupported on all elements sharing thedefining vertex
I Edge based basis functions aresupported on the two elements sharingthe defining edge
(Need to care aboutorientation of edges!)
I Cell based basis functions are only supported on the definingelement
I There can only be nonzero entries in the system matrix, if thesupport of two basis functions overlaps.
Global vector of basis functions
I Collect globally vertex, edge and cellbased basis functions (ΦV ,ΦE ,ΦC )
I Vertex based basis functions aresupported on all elements sharing thedefining vertex
I Edge based basis functions aresupported on the two elements sharingthe defining edge
(Need to care aboutorientation of edges!)
I Cell based basis functions are only supported on the definingelement
I There can only be nonzero entries in the system matrix, if thesupport of two basis functions overlaps.
Global vector of basis functions
I Collect globally vertex, edge and cellbased basis functions (ΦV ,ΦE ,ΦC )
I Vertex based basis functions aresupported on all elements sharing thedefining vertex
I Edge based basis functions aresupported on the two elements sharingthe defining edge
(Need to care aboutorientation of edges!)
I Cell based basis functions are only supported on the definingelement
I There can only be nonzero entries in the system matrix, if thesupport of two basis functions overlaps.
Global vector of basis functions
-1.0
-0.5
0.0
0.5
1.0-1.0
-0.5
0.0
0.5
1.0
-0.2
-0.1
0.0
0.1
0.2
I Collect globally vertex, edge and cellbased basis functions (ΦV ,ΦE ,ΦC )
I Vertex based basis functions aresupported on all elements sharing thedefining vertex
I Edge based basis functions aresupported on the two elements sharingthe defining edge (Need to care aboutorientation of edges!)
I Cell based basis functions are only supported on the definingelement
I There can only be nonzero entries in the system matrix, if thesupport of two basis functions overlaps.
Global vector of basis functions
I Collect globally vertex, edge and cellbased basis functions (ΦV ,ΦE ,ΦC )
I Vertex based basis functions aresupported on all elements sharing thedefining vertex
I Edge based basis functions aresupported on the two elements sharingthe defining edge (Need to care aboutorientation of edges!)
I Cell based basis functions are only supported on the definingelement
I There can only be nonzero entries in the system matrix, if thesupport of two basis functions overlaps.
Global vector of basis functions
I Collect globally vertex, edge and cellbased basis functions (ΦV ,ΦE ,ΦC )
I Vertex based basis functions aresupported on all elements sharing thedefining vertex
I Edge based basis functions aresupported on the two elements sharingthe defining edge (Need to care aboutorientation of edges!)
I Cell based basis functions are only supported on the definingelement
I There can only be nonzero entries in the system matrix, if thesupport of two basis functions overlaps.
The system matrix on the reference element
I A = (a(φi , φj)i ,j with a(u, v) =∫
Ω∇u(x , y)∇v(x , y) d(x , y)
I All basis functions are tensor products: φi (x , y) = fi (x)gi (y),hence
∇φi (x , y) = (f ′i (x)gi (y), fi (x)g ′i (y))
and a(φi (x , y), φj(x , y)) =
=
∫Q
(f ′i (x)f ′j (x)gi (y)gj(y) + fi (x)fj(x)g ′i (y)g ′j (y)
)d(x , y)
=
∫ 1
−1f ′i (x)f ′j (x) dx
∫ 1
−1gi (y)gj(y) dy
+
∫ 1
−1fi (x)fj(x) dx
∫ 1
−1g ′i (y)g ′j (y)) dy
I combination of one-dimensional bilinear forms!
The system matrix on the reference element
I A = (a(φi , φj)i ,j with a(u, v) =∫
Ω∇u(x , y)∇v(x , y) d(x , y)
I All basis functions are tensor products: φi (x , y) = fi (x)gi (y),hence
∇φi (x , y) = (f ′i (x)gi (y), fi (x)g ′i (y))
and a(φi (x , y), φj(x , y)) =
=
∫Q
(f ′i (x)f ′j (x)gi (y)gj(y) + fi (x)fj(x)g ′i (y)g ′j (y)
)d(x , y)
=
∫ 1
−1f ′i (x)f ′j (x) dx
∫ 1
−1gi (y)gj(y) dy
+
∫ 1
−1fi (x)fj(x) dx
∫ 1
−1g ′i (y)g ′j (y)) dy
I combination of one-dimensional bilinear forms!
The system matrix on the reference element
I A = (a(φi , φj)i ,j with a(u, v) =∫
Ω∇u(x , y)∇v(x , y) d(x , y)
I All basis functions are tensor products: φi (x , y) = fi (x)gi (y),hence
∇φi (x , y) = (f ′i (x)gi (y), fi (x)g ′i (y))
and a(φi (x , y), φj(x , y)) =
=
∫Q
(f ′i (x)f ′j (x)gi (y)gj(y) + fi (x)fj(x)g ′i (y)g ′j (y)
)d(x , y)
=
∫ 1
−1f ′i (x)f ′j (x) dx
∫ 1
−1gi (y)gj(y) dy
+
∫ 1
−1fi (x)fj(x) dx
∫ 1
−1g ′i (y)g ′j (y)) dy
I combination of one-dimensional bilinear forms!
The system matrix on the reference element
I A = (a(φi , φj)i ,j with a(u, v) =∫
Ω∇u(x , y)∇v(x , y) d(x , y)
I All basis functions are tensor products: φi (x , y) = fi (x)gi (y),hence
∇φi (x , y) = (f ′i (x)gi (y), fi (x)g ′i (y))
and a(φi (x , y), φj(x , y)) =
=
∫Q
(f ′i (x)f ′j (x)gi (y)gj(y) + fi (x)fj(x)g ′i (y)g ′j (y)
)d(x , y)
=
∫ 1
−1f ′i (x)f ′j (x) dx
∫ 1
−1gi (y)gj(y) dy
+
∫ 1
−1fi (x)fj(x) dx
∫ 1
−1g ′i (y)g ′j (y)) dy
I combination of one-dimensional bilinear forms!
The system matrix on the reference element
I A = (a(φi , φj)i ,j with a(u, v) =∫
Ω∇u(x , y)∇v(x , y) d(x , y)
I All basis functions are tensor products: φi (x , y) = fi (x)gi (y),hence
∇φi (x , y) = (f ′i (x)gi (y), fi (x)g ′i (y))
and a(φi (x , y), φj(x , y)) =
=
∫Q
(f ′i (x)f ′j (x)gi (y)gj(y) + fi (x)fj(x)g ′i (y)g ′j (y)
)d(x , y)
=
∫ 1
−1f ′i (x)f ′j (x) dx
∫ 1
−1gi (y)gj(y) dy
+
∫ 1
−1fi (x)fj(x) dx
∫ 1
−1g ′i (y)g ′j (y)) dy
I combination of one-dimensional bilinear forms!
The element system matrix for p = 3
The matrix can again be subdivided into block according to thedifferent types of basis functions: AVV AVE AVC
AEV AEE AEC
ACV ACE ACC
and if the basis using integrated Legendre polynomials is used, thenthe nonzero pattern is as follows...
The element system matrix for p = 3
∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗
∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗
∗ ∗ ∗ ∗ ∗∗ ∗ ∗ ∗ ∗
The element system matrix for p = 7
1 20 40 64
1
20
40
64
1 20 40 64
1
20
40
64
Efficient computations: recurrence relations
I FE-basis on quadrilaterals defined as tensor products of 1Dbasis functions
I Hierarchic 1D basis functions defined using orthogonalpolynomials
I Orthogonal polynomials φn(x) satisfy three term recurrences
φn(x) = (anx + bn)φn−1(x) + cnφn−2(x), n ≥ 1.
So do integrated Legendre polynomials!
I Derivatives of orthogonal polynomials also satisfy three termrecurrences.
I Building blocks of system matrices are integrals of the form∫Iφi (x)ψj(x) dx ,
where φi and ψj are orthogonal polynomials (satisfy threeterm recurrences).
Efficient computations: recurrence relations
I FE-basis on quadrilaterals defined as tensor products of 1Dbasis functions
I Hierarchic 1D basis functions defined using orthogonalpolynomials
I Orthogonal polynomials φn(x) satisfy three term recurrences
φn(x) = (anx + bn)φn−1(x) + cnφn−2(x), n ≥ 1.
So do integrated Legendre polynomials!
I Derivatives of orthogonal polynomials also satisfy three termrecurrences.
I Building blocks of system matrices are integrals of the form∫Iφi (x)ψj(x) dx ,
where φi and ψj are orthogonal polynomials (satisfy threeterm recurrences).
Efficient computations: recurrence relations
I FE-basis on quadrilaterals defined as tensor products of 1Dbasis functions
I Hierarchic 1D basis functions defined using orthogonalpolynomials
I Orthogonal polynomials φn(x) satisfy three term recurrences
φn(x) = (anx + bn)φn−1(x) + cnφn−2(x), n ≥ 1.
So do integrated Legendre polynomials!
I Derivatives of orthogonal polynomials also satisfy three termrecurrences.
I Building blocks of system matrices are integrals of the form∫Iφi (x)ψj(x) dx ,
where φi and ψj are orthogonal polynomials (satisfy threeterm recurrences).
Efficient computations: recurrence relations
I FE-basis on quadrilaterals defined as tensor products of 1Dbasis functions
I Hierarchic 1D basis functions defined using orthogonalpolynomials
I Orthogonal polynomials φn(x) satisfy three term recurrences
φn(x) = (anx + bn)φn−1(x) + cnφn−2(x), n ≥ 1.
So do integrated Legendre polynomials!
I Derivatives of orthogonal polynomials also satisfy three termrecurrences.
I Building blocks of system matrices are integrals of the form∫Iφi (x)ψj(x) dx ,
where φi and ψj are orthogonal polynomials (satisfy threeterm recurrences).
Efficient computations: recurrence relations
I FE-basis on quadrilaterals defined as tensor products of 1Dbasis functions
I Hierarchic 1D basis functions defined using orthogonalpolynomials
I Orthogonal polynomials φn(x) satisfy three term recurrences
φn(x) = (anx + bn)φn−1(x) + cnφn−2(x), n ≥ 1.
So do integrated Legendre polynomials!
I Derivatives of orthogonal polynomials also satisfy three termrecurrences.
I Building blocks of system matrices are integrals of the form∫Iφi (x)ψj(x) dx ,
where φi and ψj are orthogonal polynomials (satisfy threeterm recurrences).
Efficient computations: recurrence relations
I Let φn(x), ψn(x) be polynomials defined by the recurrences
φn(x) = (anx + bn)φn−1(x) + cnφn−2(x),
andψn(x) = (αnx + βn)ψn−1(x) + γnψn−2(x).
I Let Pi ,j(x) = φi (x)ψj(x), then
Pi ,j(x) = Ai ,jPi−1,j+1(x) + Bi ,jPi−1,j(x)
+ Ci ,jPi−1,j−1(x) + Di ,jPi−2,j(x),i - 2 i - 1 i
j - 1
j
j + 1
with
Ai ,j = ai/αj+1, Bi ,j = (biαj+1 − aiβj+1)/αj+1,
Ci ,j = −aiγj+1/αj+1, Di ,j = ci .
Efficient computations: recurrence relations
I Let φn(x), ψn(x) be polynomials defined by the recurrences
φn(x) = (anx + bn)φn−1(x) + cnφn−2(x),
andψn(x) = (αnx + βn)ψn−1(x) + γnψn−2(x).
I Let Pi ,j(x) = φi (x)ψj(x), then
Pi ,j(x) = Ai ,jPi−1,j+1(x) + Bi ,jPi−1,j(x)
+ Ci ,jPi−1,j−1(x) + Di ,jPi−2,j(x),i - 2 i - 1 i
j - 1
j
j + 1
with
Ai ,j = ai/αj+1, Bi ,j = (biαj+1 − aiβj+1)/αj+1,
Ci ,j = −aiγj+1/αj+1, Di ,j = ci .
Efficient computations: recurrence relations
I We have an x-free recurrence
Pi,j(x) = Ai,jPi−1,j+1(x)+Bi,jPi−1,j(x)+Ci,jPi−1,j−1(x)+Di,jPi−2,j(x),
for Pi ,j(x) = φi (x)ψj(x).
I It can be multiplied by any function f (x) and one mayintegrate over I and the recurrence remains valid, i.e.,
Mi ,j = Ai ,jMi−1,j+1 + Bi ,jMi−1,j + Ci ,jMi−1,j−1 + Di ,jMi−2,j ,
for Mi ,j =∫I f (x)φi (x)ψj(x) dx .
Efficient computations: recurrence relations
I We have an x-free recurrence
Pi,j(x) = Ai,jPi−1,j+1(x)+Bi,jPi−1,j(x)+Ci,jPi−1,j−1(x)+Di,jPi−2,j(x),
for Pi ,j(x) = φi (x)ψj(x).
I It can be multiplied by any function f (x) and one mayintegrate over I and the recurrence remains valid, i.e.,
Mi ,j = Ai ,jMi−1,j+1 + Bi ,jMi−1,j + Ci ,jMi−1,j−1 + Di ,jMi−2,j ,
for Mi ,j =∫I f (x)φi (x)ψj(x) dx .
Efficient computations: sum factorizationI Usually the bilinear forms a(u, v) building the system matrix A
are evaluated using numerical integration:
a(φi ,j , φk,l) =
∫ ∫Q
C (x , y)φi (x)ψj(y)φk(x)ψl(y) d(x , y)
'∑α,β
wαwβC (xα, yβ)φi (xα)ψj(yβ)φk(xα)ψl(yβ),
whereI C (·, ·) is a coefficient function (contains also transformation
from the actual element in the mesh to the reference element),I wγ are the quadrature weights and xα, yβ are the quadrature
points.
I For the approximate solution of the linear system Au = f aniterative scheme is used
u(k+1) = f − Au(k),
hence not fast assemblance of the matrix is needed, but fastapplication.
Efficient computations: sum factorizationI Usually the bilinear forms a(u, v) building the system matrix A
are evaluated using numerical integration:
a(φi ,j , φk,l) =
∫ ∫Q
C (x , y)φi (x)ψj(y)φk(x)ψl(y) d(x , y)
'∑α,β
wαwβC (xα, yβ)φi (xα)ψj(yβ)φk(xα)ψl(yβ),
whereI C (·, ·) is a coefficient function (contains also transformation
from the actual element in the mesh to the reference element),I wγ are the quadrature weights and xα, yβ are the quadrature
points.
I For the approximate solution of the linear system Au = f aniterative scheme is used
u(k+1) = f − Au(k),
hence not fast assemblance of the matrix is needed, but fastapplication.
Efficient computation: sum factorization
Let’s write the solution vector u with two indices, then(Au)k,l'∑α,β,i ,j
wαwβC (xα, yβ)φi (xα)ψj(yβ)φk(xα)ψl(yβ)ui ,j
where each summation is O(p) with maximal polynomial degree p.
M(1)α,j =
∑i
φi (xα)ui ,j
M(2)α,β =
∑j
M(1)α,j ψj(yβ)
M(3)β,k =
∑α
M(2)α,βwαC (xα, yβ)φk(xα)
(Au)k,l'∑β
M(3)β,kwβψl(yβ)
Efficient computation: sum factorization
Let’s write the solution vector u with two indices, then(Au)k,l'∑α,β,i ,j
wαwβC (xα, yβ)φi (xα)ψj(yβ)φk(xα)ψl(yβ)ui ,j
where each summation is O(p) with maximal polynomial degree p.
M(1)α,j =
∑i
φi (xα)ui ,j
M(2)α,β =
∑j
M(1)α,j ψj(yβ)
M(3)β,k =
∑α
M(2)α,βwαC (xα, yβ)φk(xα)
(Au)k,l'∑β
M(3)β,kwβψl(yβ)
Efficient computation: sum factorization
Let’s write the solution vector u with two indices, then(Au)k,l'∑α,β,i ,j
wαwβC (xα, yβ)φi (xα)ψj(yβ)φk(xα)ψl(yβ)ui ,j
where each summation is O(p) with maximal polynomial degree p.
M(1)α,j =
∑i
φi (xα)ui ,j
M(2)α,β =
∑j
M(1)α,j ψj(yβ)
M(3)β,k =
∑α
M(2)α,βwαC (xα, yβ)φk(xα)
(Au)k,l'∑β
M(3)β,kwβψl(yβ)
Efficient computation: sum factorization
Let’s write the solution vector u with two indices, then(Au)k,l'∑α,β,i ,j
wαwβC (xα, yβ)φi (xα)ψj(yβ)φk(xα)ψl(yβ)ui ,j
where each summation is O(p) with maximal polynomial degree p.
M(1)α,j =
∑i
φi (xα)ui ,j
M(2)α,β =
∑j
M(1)α,j ψj(yβ)
M(3)β,k =
∑α
M(2)α,βwαC (xα, yβ)φk(xα)
(Au)k,l'∑β
M(3)β,kwβψl(yβ)
Efficient computation: sum factorization
Let’s write the solution vector u with two indices, then(Au)k,l'∑α,β,i ,j
wαwβC (xα, yβ)φi (xα)ψj(yβ)φk(xα)ψl(yβ)ui ,j
where each summation is O(p) with maximal polynomial degree p.
M(1)α,j =
∑i
φi (xα)ui ,j
M(2)α,β =
∑j
M(1)α,j ψj(yβ)
M(3)β,k =
∑α
M(2)α,βwαC (xα, yβ)φk(xα)
(Au)k,l'∑β
M(3)β,kwβψl(yβ)
Hierarchic high order basis functions on triangles
On quadrilateral elements the basis func-tions were defined exploiting the tensorproduct structure:
φi ,j(x , y) = ϕi (x)ψj(y),
with i , j ≥ 0 and x , y ∈ [−1, 1]. H-1,-1L H1,-1L
H1,1LH-1,1L
H-1,-1L H1,-1L
H0,1LThe basis functions on the reference tri-angle T are defined by collapsing thequadrilateral to the triangle:
φi ,j(x , y) = ϕi
(2x
1−y
)(1−y
2
)iψj(y).
Hierarchic high order basis functions on triangles
On quadrilateral elements the basis func-tions were defined exploiting the tensorproduct structure:
φi ,j(x , y) = ϕi (x)ψj(y),
with i , j ≥ 0 and x , y ∈ [−1, 1]. H-1,-1L H1,-1L
H1,1LH-1,1L
H-1,-1L H1,-1L
H0,1LThe basis functions on the reference tri-angle T are defined by collapsing thequadrilateral to the triangle:
φi ,j(x , y) = ϕi
(2x
1−y
)(1−y
2
)iψj(y).
Integral over reference triangle
Let φi ,j(x , y) = ϕi
(2x
1−y
)(1−y
2
)iψj(y), then by means of the
substitution z = 2x1−y we decouple the integrals:
∫Tφi ,j(x , y)φk,l(x , y) d(x , y) =
∫ 1
−1ϕi (x)ϕk(x) dx
×∫ 1
−1
(1−z
2
)i+k+1ψj(z)ψl(z) dz .
I Set ϕi (x) = Pi (x) (Legendre polynomials)
I Set ψj(y) = P(2i+1,0)j (y), the Jacobi polynomials orthogonal
w.r.t. the weight function(
1−y2
)2i+1, i.e.,
∫ 1
−1
(1−y
2
)2i+1P
(2i+1,0)j (y)P
(2i+1,0)l (y) dy = hijδj ,l
Integral over reference triangle
Let φi ,j(x , y) = ϕi
(2x
1−y
)(1−y
2
)iψj(y), then by means of the
substitution z = 2x1−y we decouple the integrals:
∫Tφi ,j(x , y)φk,l(x , y) d(x , y) =
∫ 1
−1Pi (x)Pk(x) dx
×∫ 1
−1
(1−z
2
)i+k+1ψj(z)ψl(z) dz .
I Set ϕi (x) = Pi (x) (Legendre polynomials)
I Set ψj(y) = P(2i+1,0)j (y), the Jacobi polynomials orthogonal
w.r.t. the weight function(
1−y2
)2i+1, i.e.,
∫ 1
−1
(1−y
2
)2i+1P
(2i+1,0)j (y)P
(2i+1,0)l (y) dy = hijδj ,l
Integral over reference triangle
Let φi ,j(x , y) = ϕi
(2x
1−y
)(1−y
2
)iψj(y), then by means of the
substitution z = 2x1−y we decouple the integrals:
∫Tφi ,j(x , y)φk,l(x , y) d(x , y) = giδi ,k
×∫ 1
−1
(1−z
2
)2i+1ψj(z)ψl(z) dz .
I Set ϕi (x) = Pi (x) (Legendre polynomials)
I Set ψj(y) = P(2i+1,0)j (y), the Jacobi polynomials orthogonal
w.r.t. the weight function(
1−y2
)2i+1, i.e.,
∫ 1
−1
(1−y
2
)2i+1P
(2i+1,0)j (y)P
(2i+1,0)l (y) dy = hijδj ,l
Integral over reference triangle
Let φi ,j(x , y) = ϕi
(2x
1−y
)(1−y
2
)iψj(y), then by means of the
substitution z = 2x1−y we decouple the integrals:
∫Tφi ,j(x , y)φk,l(x , y) d(x , y) = giδi ,k
×∫ 1
−1
(1−z
2
)2i+1ψj(z)ψl(z) dz .
I Set ϕi (x) = Pi (x) (Legendre polynomials)
I Set ψj(y) = P(2i+1,0)j (y), the Jacobi polynomials orthogonal
w.r.t. the weight function(
1−y2
)2i+1, i.e.,
∫ 1
−1
(1−y
2
)2i+1P
(2i+1,0)j (y)P
(2i+1,0)l (y) dy = hijδj ,l
Integral over reference triangle
Let φi ,j(x , y) = ϕi
(2x
1−y
)(1−y
2
)iψj(y), then by means of the
substitution z = 2x1−y we decouple the integrals:∫
Tφi ,j(x , y)φk,l(x , y) d(x , y) = giδi ,k
×∫ 1
−1
(1−z
2
)2i+1P
(2i+1,0)j (z)P
(2i+1,0)l (z) dz .
I Set ϕi (x) = Pi (x) (Legendre polynomials)
I Set ψj(y) = P(2i+1,0)j (y), the Jacobi polynomials orthogonal
w.r.t. the weight function(
1−y2
)2i+1, i.e.,
∫ 1
−1
(1−y
2
)2i+1P
(2i+1,0)j (y)P
(2i+1,0)l (y) dy = hijδj ,l
Integral over reference triangle
Let φi ,j(x , y) = ϕi
(2x
1−y
)(1−y
2
)iψj(y), then by means of the
substitution z = 2x1−y we decouple the integrals:∫
Tφi ,j(x , y)φk,l(x , y) d(x , y) = gihijδi ,kδj ,l
Dubiner basis
I Set ϕi (x) = Pi (x) (Legendre polynomials)
I Set ψj(y) = P(2i+1,0)j (y), the Jacobi polynomials orthogonal
w.r.t. the weight function(
1−y2
)2i+1, i.e.,
∫ 1
−1
(1−y
2
)2i+1P
(2i+1,0)j (y)P
(2i+1,0)l (y) dy = hijδj ,l
Sparsity optimized basis functions for simplices
I Dubiner: L2-orthogonal basis functions on triangles (suitablychosen Jacobi polynomials)
I Beuchler+Schoberl: sparse system matrices for H1 ontriangles (suitably chosen integrated Jacobi polynomials)
I Beuchler + VP: extension of the basis construction in H1 totetrahedra
I Beuchler+VP+Zaglmayr: basis functions for H(div) andH(curl), where
I construction principle follows ZaglmayrI exploit products of suitably chosen integrated Jacobi
polynomials
I Proof of sparsity in the latter three cases: exact evaluation ofthe integrals using symbolic computation
Sparsity optimized basis functions for simplices
I Dubiner: L2-orthogonal basis functions on triangles (suitablychosen Jacobi polynomials)
I Beuchler+Schoberl: sparse system matrices for H1 ontriangles (suitably chosen integrated Jacobi polynomials)
I Beuchler + VP: extension of the basis construction in H1 totetrahedra
I Beuchler+VP+Zaglmayr: basis functions for H(div) andH(curl), where
I construction principle follows ZaglmayrI exploit products of suitably chosen integrated Jacobi
polynomials
I Proof of sparsity in the latter three cases: exact evaluation ofthe integrals using symbolic computation
Sparsity optimized basis functions for simplices
I Dubiner: L2-orthogonal basis functions on triangles (suitablychosen Jacobi polynomials)
I Beuchler+Schoberl: sparse system matrices for H1 ontriangles (suitably chosen integrated Jacobi polynomials)
I Beuchler + VP: extension of the basis construction in H1 totetrahedra
I Beuchler+VP+Zaglmayr: basis functions for H(div) andH(curl), where
I construction principle follows ZaglmayrI exploit products of suitably chosen integrated Jacobi
polynomials
I Proof of sparsity in the latter three cases: exact evaluation ofthe integrals using symbolic computation
Sparsity optimized basis functions for simplices
I Dubiner: L2-orthogonal basis functions on triangles (suitablychosen Jacobi polynomials)
I Beuchler+Schoberl: sparse system matrices for H1 ontriangles (suitably chosen integrated Jacobi polynomials)
I Beuchler + VP: extension of the basis construction in H1 totetrahedra
I Beuchler+VP+Zaglmayr: basis functions for H(div) andH(curl), where
I construction principle follows ZaglmayrI exploit products of suitably chosen integrated Jacobi
polynomials
I Proof of sparsity in the latter three cases: exact evaluation ofthe integrals using symbolic computation
Sparsity optimized basis functions for simplices
I Dubiner: L2-orthogonal basis functions on triangles (suitablychosen Jacobi polynomials)
I Beuchler+Schoberl: sparse system matrices for H1 ontriangles (suitably chosen integrated Jacobi polynomials)
I Beuchler + VP: extension of the basis construction in H1 totetrahedra
I Beuchler+VP+Zaglmayr: basis functions for H(div) andH(curl), where
I construction principle follows ZaglmayrI exploit products of suitably chosen integrated Jacobi
polynomials
I Proof of sparsity in the latter three cases: exact evaluation ofthe integrals using symbolic computation
Jacobi and integrated Jacobi polynomials
I For α > −1, −1 ≤ x ≤ 1 and n ≥ 0 we denote by P(α,0)n (x)
the nth Jacobi polynomial orthogonal w.r.t. the inner product
〈f , g〉L2α(−1,1)
∫ 1
−1
(1−x
2
)αf (x)g(x) dx .
I We define the nth integrated Jacobi polynomial as
pαn (x) =
∫ x
−1P
(α,0)n−1 (s) ds.
I Legendre polynomials Pn(x) = P(0,0)n (x)
Jacobi and integrated Jacobi polynomials
I For α > −1, −1 ≤ x ≤ 1 and n ≥ 0 we denote by P(α,0)n (x)
the nth Jacobi polynomial orthogonal w.r.t. the inner product
〈f , g〉L2α(−1,1)
∫ 1
−1
(1−x
2
)αf (x)g(x) dx .
I We define the nth integrated Jacobi polynomial as
pαn (x) =
∫ x
−1P
(α,0)n−1 (s) ds.
I Legendre polynomials Pn(x) = P(0,0)n (x)
Jacobi and integrated Jacobi polynomials
I For α > −1, −1 ≤ x ≤ 1 and n ≥ 0 we denote by P(α,0)n (x)
the nth Jacobi polynomial orthogonal w.r.t. the inner product
〈f , g〉L2α(−1,1)
∫ 1
−1
(1−x
2
)αf (x)g(x) dx .
I We define the nth integrated Jacobi polynomial as
pαn (x) =
∫ x
−1P
(α,0)n−1 (s) ds.
I Legendre polynomials Pn(x) = P(0,0)n (x)
High order basis functions on tetrahedra
I vertex, edge, face, and cell basedbasis functions
I usually defined by collapsing ahexahedron to a tetrahedron
A=H-1,-1,-1L B=H1,-1,-1L
C=H0,1,-1L
D=H0,0,1L
F1
F2 F3
F1=D ABCF2=D ACDF3=D BCDF4=D ABD
Example: Cell based basis function for H1:
φijk(x , y , z) = p0i
(4x
1− 2y − z
)(1− 2y − z)i p2i
j
(2y
1− z
)× (1− z)j p2i+2j
k (z)
High order basis functions on tetrahedra
I vertex, edge, face, and cell basedbasis functions
I usually defined by collapsing ahexahedron to a tetrahedron
A=H-1,-1,-1L B=H1,-1,-1L
C=H0,1,-1L
D=H0,0,1L
F1
F2 F3
F1=D ABCF2=D ACDF3=D BCDF4=D ABD
Example: Cell based basis function for H1:
φijk(x , y , z) = p0i
(4x
1− 2y − z
)(1− 2y − z)i p2i
j
(2y
1− z
)× (1− z)j p2i+2j
k (z)
System matrix
I The FE-matrix that we consider is built from
a(u, v) =
∫T
uv d(x , y , z) +
∫T
(∇u)TC∇v d(x , y , z),
where C is a constant coefficient matrix.
I The results we obtain are of the form:
Let A = (a(φijk , φlmn))ijklmn, then the entries of the system
matrix are zero if |i − l | > 2 or |i + j − l − m| > 3 or|i + j + k − l −m − n| > 2.
I The results are proven by explicitly evaluating the integralsusing rewriting.
System matrix
I The FE-matrix that we consider is built from
a(u, v) =
∫T
uv d(x , y , z) +
∫T
(∇u)TC∇v d(x , y , z),
where C is a constant coefficient matrix.
I The results we obtain are of the form:
Let A = (a(φijk , φlmn))ijklmn, then the entries of the system
matrix are zero if |i − l | > 2 or |i + j − l − m| > 3 or|i + j + k − l −m − n| > 2.
I The results are proven by explicitly evaluating the integralsusing rewriting.
System matrix
I The FE-matrix that we consider is built from
a(u, v) =
∫T
uv d(x , y , z) +
∫T
(∇u)TC∇v d(x , y , z),
where C is a constant coefficient matrix.
I The results we obtain are of the form:
Let A = (a(φijk , φlmn))ijklmn, then the entries of the system
matrix are zero if |i − l | > 2 or |i + j − l − m| > 3 or|i + j + k − l −m − n| > 2.
I The results are proven by explicitly evaluating the integralsusing rewriting.
Example for evaluation
After decoupling the integrals for computing∫T
φijk(x , y , z)φlmn(x , y , z) d(x , y , z)
are
1∫−1
p0i (x)p
0l (x) dx
1∫−1
p2ij (y)p
2lm(y)(1− y)i+l+1 dy
1∫−1
p2i+2jk (z)p2l+2m
n (z)(1− z)i+j+l+m+2 dz
I application of 2 rewrite rules and evaluation of 4 integrals
I application of 6 rewrite rules and evaluation of 40 integrals
I application of 14 rewrite rules and evaluation of 147 integrals
In total 189 nonzero matrix entries
Example for evaluation
After decoupling the integrals for computing∫T
φijk(x , y , z)φlmn(x , y , z) d(x , y , z)
are
1∫−1
p0i (x)p
0l (x) dx
1∫−1
p2ij (y)p
2lm(y)(1− y)i+l+1 dy
1∫−1
p2i+2jk (z)p2l+2m
n (z)(1− z)i+j+l+m+2 dz
I application of 2 rewrite rules and evaluation of 4 integrals
I application of 6 rewrite rules and evaluation of 40 integrals
I application of 14 rewrite rules and evaluation of 147 integrals
In total 189 nonzero matrix entries
Example for evaluation
After decoupling the integrals for computing∫T
φijk(x , y , z)φlmn(x , y , z) d(x , y , z)
are
1∫−1
p0i (x)p
0l (x) dx
1∫−1
p2ij (y)p
2lm(y)(1− y)i+l+1 dy
1∫−1
p2i+2jk (z)p2l+2m
n (z)(1− z)i+j+l+m+2 dz
I application of 2 rewrite rules and evaluation of 4 integrals
I application of 6 rewrite rules and evaluation of 40 integrals
I application of 14 rewrite rules and evaluation of 147 integrals
In total 189 nonzero matrix entries
Example for evaluation
After decoupling the integrals for computing∫T
φijk(x , y , z)φlmn(x , y , z) d(x , y , z)
are
1∫−1
p0i (x)p
0l (x) dx
1∫−1
p2ij (y)p
2lm(y)(1− y)i+l+1 dy
1∫−1
p2i+2jk (z)p2l+2m
n (z)(1− z)i+j+l+m+2 dz
I application of 2 rewrite rules and evaluation of 4 integrals
I application of 6 rewrite rules and evaluation of 40 integrals
I application of 14 rewrite rules and evaluation of 147 integrals
In total 189 nonzero matrix entries
Example for evaluation
After decoupling the integrals for computing∫T
φijk(x , y , z)φlmn(x , y , z) d(x , y , z)
are
1∫−1
p0i (x)p
0l (x) dx
1∫−1
p2ij (y)p
2lm(y)(1− y)i+l+1 dy
1∫−1
p2i+2jk (z)p2l+2m
n (z)(1− z)i+j+l+m+2 dz
I application of 2 rewrite rules and evaluation of 4 integrals
I application of 6 rewrite rules and evaluation of 40 integrals
I application of 14 rewrite rules and evaluation of 147 integrals
In total 189 nonzero matrix entries
Example for evaluation
After decoupling the integrals for computing∫T
φijk(x , y , z)φlmn(x , y , z) d(x , y , z)
are
1∫−1
p0i (x)p
0l (x) dx
1∫−1
p2ij (y)p
2lm(y)(1− y)i+l+1 dy
1∫−1
p2i+2jk (z)p2l+2m
n (z)(1− z)i+j+l+m+2 dz
I application of 2 rewrite rules and evaluation of 4 integrals
I application of 6 rewrite rules and evaluation of 40 integrals
I application of 14 rewrite rules and evaluation of 147 integrals
In total 189 nonzero matrix entries
Sparsity pattern for H(div), p = 15
I using standard Legendre-type polynomials
element mass matrix element stiffness matrix
Sparsity pattern for H(div), p = 15
I using the Jacobi-type polynomials with optimized parameters
element mass matrix element stiffness matrix
Fast assembling of the system matrices
I Lehrenfeld+Koutschan+Schoberl: using mixed relations ofthe form
R∑r=0
cr (i , j)φi+r1,j+r2(x , y) =S∑
s=0
Dxφi+s1,j+s2(x , y)
I Combining recurrences for Jacobi polynomials pαn (x), pαn (x)and sum factorization techniques [Beuchler et al]
I Computing recurrences for the explicit matrix entries
Local Fourier analysis (Takacs + VP)
I standard tool to analyze the convergence behaviour of anumerical method
I Model problem: optimization problem constrained to a partialdifferential equationFind a state y and a control u that minimize
J(y , u) := 12‖y − yD‖2
L2(Ω) + α2 ‖u‖
2L2(Ω),
subject to −∆y = u in the domain Ω.
I solution method: Finite Element Method (FEM) with amultigrid-solver (which ultimately means solving a large scalelinear system)
I robust with respect to parameters such as mesh-size andregularization parameters
Local Fourier analysis (Takacs + VP)
I standard tool to analyze the convergence behaviour of anumerical method
I Model problem: optimization problem constrained to a partialdifferential equationFind a state y and a control u that minimize
J(y , u) := 12‖y − yD‖2
L2(Ω) + α2 ‖u‖
2L2(Ω),
subject to −∆y = u in the domain Ω.
I solution method: Finite Element Method (FEM) with amultigrid-solver (which ultimately means solving a large scalelinear system)
I robust with respect to parameters such as mesh-size andregularization parameters
Local Fourier analysis (Takacs + VP)
I standard tool to analyze the convergence behaviour of anumerical method
I Model problem: optimization problem constrained to a partialdifferential equationFind a state y and a control u that minimize
J(y , u) := 12‖y − yD‖2
L2(Ω) + α2 ‖u‖
2L2(Ω),
subject to −∆y = u in the domain Ω.
I solution method: Finite Element Method (FEM) with amultigrid-solver (which ultimately means solving a large scalelinear system)
I robust with respect to parameters such as mesh-size andregularization parameters
Local Fourier analysis (Takacs + VP)
I standard tool to analyze the convergence behaviour of anumerical method
I Model problem: optimization problem constrained to a partialdifferential equationFind a state y and a control u that minimize
J(y , u) := 12‖y − yD‖2
L2(Ω) + α2 ‖u‖
2L2(Ω),
subject to −∆y = u in the domain Ω.
I solution method: Finite Element Method (FEM) with amultigrid-solver (which ultimately means solving a large scalelinear system)
I robust with respect to parameters such as mesh-size andregularization parameters
Local Fourier Analysis
I Given: Iterative procedure of the form
x (k+1) = Ax (k)
the convergence rate is related to the matrix norm of A whichcan be estimated by the spectral radius, i.e., the largesteigenvalue in absolute value
I typically rates are obtained only by numerical interpolation
I symbolic local Fourier analysis: exact bounds
Local Fourier Analysis
I Given: Iterative procedure of the form
x (k+1) = Ax (k)
the convergence rate is related to the matrix norm of A whichcan be estimated by the spectral radius, i.e., the largesteigenvalue in absolute value
I typically rates are obtained only by numerical interpolation
I symbolic local Fourier analysis: exact bounds
Local Fourier Analysis
I Given: Iterative procedure of the form
x (k+1) = Ax (k)
the convergence rate is related to the matrix norm of A whichcan be estimated by the spectral radius, i.e., the largesteigenvalue in absolute value
I typically rates are obtained only by numerical interpolation
I symbolic local Fourier analysis: exact bounds
FEM and multigrid
I Multigrid methods operate on two (ormore) grids
I One step in a multigrid method consistsof
I (pre)smoothing steps
↓ restriction ↓
I coarse grid correction
↓ prolongation ↓
I (post)smoothing steps
FEM and multigrid
I Multigrid methods operate on two (ormore) grids
I One step in a multigrid method consistsof
I (pre)smoothing steps
↓ restriction ↓
I coarse grid correction
↓ prolongation ↓
I (post)smoothing steps
FEM and multigrid
I Multigrid methods operate on two (ormore) grids
I One step in a multigrid method consistsof
I (pre)smoothing steps
↓ restriction ↓
I coarse grid correction
↓ prolongation ↓
I (post)smoothing steps
Bound the convergence rate (2D)
Given: matrix A(q, c1, c2, η) ∈ R8×8 with 0 < q < 1 and
(c1, c2, η) ∈ Ω = (c1, c2, η) | 0 ≤ c1 ≤ c2 < 1 ∧ η > 0,
where ci = cos(θi ) for some frequencies θi and η = h4/α withmesh-size h and regularization parameter α.
Find: bound B(q) for the maximal eigenvalue λmax(q, c1, c2, η)of A over (0, 1)2 × Ω.
The Matrix A
A1,1 0 A1,3 A1,4 A1,5 A1,6 A1,7 A1,8
0 A2,2 A2,3 A2,4 A2,5 A2,6 A2,7 A2,8
A3,1 A3,2 A3,3 0 A3,5 A3,6 A3,7 A3,8
A4,1 A4,2 0 A4,4 A4,5 A4,6 A4,7 A4,8
A5,1 A5,2 A5,3 A5,4 A5,5 0 A5,7 A5,8
A6,1 A6,2 A6,3 A6,4 0 A6,6 A6,7 A6,8
A7,1 A7,2 A7,3 A7,4 A7,5 A7,6 A7,7 0A8,1 A8,2 A8,3 A8,4 A8,5 A8,6 0 A8,8
Common denominator of the matrix entries:
D =256(16c4
2 c41η + 16c2
2 c41η + 4c4
1η + 16c42 c
21η + 16c2
2 c21η + 4c2
1η
+ 4c42η + 4c2
2η + 144c42 c
41 − 72c2
2 c41 + 9c4
1 − 72c42 c
21 − 126c2
2 c21
+36c21 + 9c4
2 + 36c22 + η + 36
)
The Matrix A
A1,1 0 A1,3 A1,4 A1,5 A1,6 A1,7 A1,8
0 A2,2 A2,3 A2,4 A2,5 A2,6 A2,7 A2,8
A3,1 A3,2 A3,3 0 A3,5 A3,6 A3,7 A3,8
A4,1 A4,2 0 A4,4 A4,5 A4,6 A4,7 A4,8
A5,1 A5,2 A5,3 A5,4 A5,5 0 A5,7 A5,8
A6,1 A6,2 A6,3 A6,4 0 A6,6 A6,7 A6,8
A7,1 A7,2 A7,3 A7,4 A7,5 A7,6 A7,7 0A8,1 A8,2 A8,3 A8,4 A8,5 A8,6 0 A8,8
Common denominator of the matrix entries:
D =256(16c4
2 c41η + 16c2
2 c41η + 4c4
1η + 16c42 c
21η + 16c2
2 c21η + 4c2
1η
+ 4c42η + 4c2
2η + 144c42 c
41 − 72c2
2 c41 + 9c4
1 − 72c42 c
21 − 126c2
2 c21
+36c21 + 9c4
2 + 36c22 + η + 36
)
The Numerator of A1,1
432q2c62 c
61 + 3q2ηc6
2 c61 + ηc6
2 c61 + 144c6
2 c61 + 1008q2c5
2 c61 + 16q2ηc5
2 c61 + 8ηc5
2 c61 + 720c5
2 c61 + 972q2c4
2 c61 +
30q2ηc42 c
61 + 26ηc4
2 c61 + 1476c4
2 c61 + 1008q2c3
2 c61 + 28q2ηc3
2 c61 + 44ηc3
2 c61 + 1584c3
2 c61 + 108q2c6
1 + 1080q2c22 c
61 +
27q2ηc22 c
61 + 41ηc2
2 c61 + 936c2
2 c61 + 12q2ηc6
1 + 4ηc61 + 576q2c2c
61 + 28q2ηc2c
61 + 20ηc2c
61 + 288c2c
61 + 36c6
1 +
1008q2c62 c
51 + 16q2ηc6
2 c51 + 8ηc6
2 c51 + 720c6
2 c51 −360q2c5
2 c51 + 80q2ηc5
2 c51 −64ηc5
2 c51 −1656c5
2 c51 −3600q2c4
2 c51 +
128q2ηc42 c
51 − 304ηc4
2 c51 − 7056c4
2 c51 − 2736q2c3
2 c51 + 80q2ηc3
2 c51 − 352ηc3
2 c51 − 5328c3
2 c51 − 720q2c5
1 −1872q2c2
2 c51 + 80q2ηc2
2 c51 − 184ηc2
2 c51 + 720c2
2 c51 + 64q2ηc5
1 − 96ηc51 − 2088q2c2c
51 + 128q2ηc2c
51 − 160ηc2c
51 +
1800c2c51 + 432c5
1 + 972q2c62 c
41 + 30q2ηc6
2 c41 + 26ηc6
2 c41 + 1476c6
2 c41 − 3600q2c5
2 c41 + 128q2ηc5
2 c41 − 304ηc5
2 c41 −
7056c52 c
41 − 5616q2c4
2 c41 + 108q2ηc4
2 c41 + 2724ηc4
2 c41 + 21168c4
2 c41 + 1584q2c3
2 c41 − 136q2ηc3
2 c41 − 1672ηc3
2 c41 −
3600c32 c
41 +648q2c4
1 +1404q2c22 c
41 −114q2ηc2
2 c41 +3114ηc2
2 c41 −15660c2
2 c41 +120q2ηc4
1 +616ηc41 −576q2c2c
41 +
152q2ηc2c41 −760ηc2c
41 −2304c2c
41 + 792c4
1 + 1008q2c62 c
31 + 28q2ηc6
2 c31 + 44ηc6
2 c31 + 1584c6
2 c31 −2736q2c5
2 c31 +
80q2ηc52 c
31 − 352ηc5
2 c31 − 5328c5
2 c31 + 1584q2c4
2 c31 − 136q2ηc4
2 c31 − 1672ηc4
2 c31 − 3600c4
2 c31 + 12384q2c3
2 c31 −
640q2ηc32 c
31 − 1936ηc3
2 c31 + 17568c3
2 c31 + 1872q2c3
1 + 5904q2c22 c
31 − 580q2ηc2
2 c31 − 1012ηc2
2 c31 + 14544c2
2 c31 +
112q2ηc31 − 528ηc3
1 + 720q2c2c31 − 16q2ηc2c
31 − 880ηc2c
31 − 1872c2c
31 − 2160c3
1 + 1080q2c62 c
21 + 27q2ηc6
2 c21 +
41ηc62 c
21 + 936c6
2 c21 − 1872q2c5
2 c21 + 80q2ηc5
2 c21 − 184ηc5
2 c21 + 720c5
2 c21 + 1404q2c4
2 c21 − 114q2ηc4
2 c21 +
3114ηc42 c
21 − 15660c4
2 c21 + 5904q2c3
2 c21 − 580q2ηc3
2 c21 − 1012ηc3
2 c21 + 14544c3
2 c21 + 108q2c2
1 − 5184q2c22 c
21 −
525q2ηc22 c
21 + 3729ηc2
2 c21 − 15552c2
2 c21 + 108q2ηc2
1 + 676ηc21 − 6624q2c2c
21 − 4q2ηc2c
21 − 460ηc2c
21 +
2880c2c21 + 6948c2
1 + 576q2c62 c1 + 28q2ηc6
2 c1 + 20ηc62 c1 + 288c6
2 c1 − 2088q2c52 c1 + 128q2ηc5
2 c1 − 160ηc52 c1 +
1800c52 c1 − 576q2c4
2 c1 + 152q2ηc42 c1 − 760ηc4
2 c1 − 2304c42 c1 + 720q2c3
2 c1 − 16q2ηc32 c1 − 880ηc3
2 c1 −1872c3
2 c1 + 1440q2c1 − 6624q2c22 c1 − 4q2ηc2
2 c1 − 460ηc22 c1 + 2880c2
2 c1 + 112q2ηc1 − 240ηc1 − 3816q2c2c1 +
176q2ηc2c1 − 400ηc2c1 − 5112c2c1 − 6048c1 + 108q2c62 + 12q2ηc6
2 + 4ηc62 + 36c6
2 − 720q2c52 + 64q2ηc5
2 −96ηc5
2 + 432c52 + 648q2c4
2 + 120q2ηc42 + 616ηc4
2 + 792c42 + 1872q2c3
2 + 112q2ηc32 − 528ηc3
2 − 2160c32 + 1728q2 +
108q2c22 + 108q2ηc2
2 + 676ηc22 + 6948c2
2 + 48q2η + 144η + 1440q2c2 + 112q2ηc2 − 240ηc2 − 6048c2 + 5184
Computational Issues
1. How to find the eigenvalues?
I symbolic interpolation in q
2. How to find the bound?
I consider the boundary of the domain to obtain a plausibleguess for the bound using CAD
3. How to prove the bound?
I use CADI necessary to split into subtasks and consider sufficient
conditions
Computational Issues
1. How to find the eigenvalues?I symbolic interpolation in q
2. How to find the bound?
I consider the boundary of the domain to obtain a plausibleguess for the bound using CAD
3. How to prove the bound?
I use CADI necessary to split into subtasks and consider sufficient
conditions
Computational Issues
1. How to find the eigenvalues?I symbolic interpolation in q
2. How to find the bound?I consider the boundary of the domain to obtain a plausible
guess for the bound using CAD
3. How to prove the bound?
I use CADI necessary to split into subtasks and consider sufficient
conditions
Computational Issues
1. How to find the eigenvalues?I symbolic interpolation in q
2. How to find the bound?I consider the boundary of the domain to obtain a plausible
guess for the bound using CAD
3. How to prove the bound?I use CAD
I necessary to split into subtasks and consider sufficientconditions
Computational Issues
1. How to find the eigenvalues?I symbolic interpolation in q
2. How to find the bound?I consider the boundary of the domain to obtain a plausible
guess for the bound using CAD
3. How to prove the bound?I use CADI necessary to split into subtasks and consider sufficient
conditions
The Eigenvalues
Using interpolation on the characteristic polynomial of the matrixwe find that the eigenvalues are
λ1 = 0, λ2 = q4, 3λ4 =1
D
(e(q2)±
√d(q2)
),
each of multiplicity 2 with q2 = q2 and e, d polynomials in c1, c2, ηand q2.
The largest eigenvalue is
λmax =1
D
(e(q2) +
√d(q2)
).
The Eigenvalues
Using interpolation on the characteristic polynomial of the matrixwe find that the eigenvalues are
λ1 = 0, λ2 = q4, 3λ4 =1
D
(e(q2)±
√d(q2)
),
each of multiplicity 2 with q2 = q2 and e, d polynomials in c1, c2, ηand q2.
The largest eigenvalue is
λmax =1
D
(e(q2) +
√d(q2)
).
The Eigenvalues
Using interpolation on the characteristic polynomial of the matrixwe find that the eigenvalues are
λ1 = 0, λ2 = q4, 3λ4 =1
D
(e(q2)±
√d(q2)
),
each of multiplicity 2 with q2 = q2 and e, d polynomials in c1, c2, ηand q2.
The largest eigenvalue is
λmax =1
D
(e(q2) +
√d(q2)
).
The Polynomial e(q2)
ηc62 c
61 + 144c6
2 c61 + 8ηc5
2 c61 + 720c5
2 c61 + 26ηc4
2 c61 + 1476c4
2 c61 + 44ηc3
2 c61 + 1584c3
2 c61 + 41ηc2
2 c61 + 936c2
2 c61 +
9ηc62 q
22c
61 +1296c6
2 q22c
61 −24ηc5
2 q22c
61 −2160c5
2 q22c
61 +138ηc4
2 q22c
61 +6372c4
2 q22c
61 −132ηc3
2 q22c
61 −4752c3
2 q22c
61 +
177ηc22 q
22c
61 + 4968c2
2 q22c
61 + 36ηq2
2c61 − 60ηc2q
22c
61 − 864c2q
22c
61 + 324q2
2c61 + 4ηc6
1 + 20ηc2c61 + 288c2c
61 +
6ηc62 q2c
61 + 864c6
2 q2c61 + 16ηc5
2 q2c61 + 1440c5
2 q2c61 + 60ηc4
2 q2c61 + 1944c4
2 q2c61 + 88ηc3
2 q2c61 + 3168c3
2 q2c61 +
54ηc22 q2c
61 +2160c2
2 q2c61 +24ηq2c
61 +40ηc2q2c
61 +576c2q2c
61 +216q2c
61 +36c6
1 +8ηc62 c
51 +720c6
2 c51 −64ηc5
2 c51 −
1656c52 c
51 −304ηc4
2 c51 −7056c4
2 c51 −352ηc3
2 c51 −5328c3
2 c51 −184ηc2
2 c51 + 720c2
2 c51 −24ηc6
2 q22c
51 −2160c6
2 q22c
51 +
32ηc52 q
22c
51 +2664c5
2 q22c
51 +272ηc4
2 q22c
51 +10224c4
2 q22c
51 +416ηc3
2 q22c
51 +3312c3
2 q22c
51 +296ηc2
2 q22c
51 −2736c2
2 q22c
51 +
32ηq22c
51 + 128ηc2q
22c
51 − 792c2q
22c
51 − 144q2
2c51 − 96ηc5
1 − 160ηc2c51 + 1800c2c
51 + 16ηc6
2 q2c51 + 1440c6
2 q2c51 +
32ηc52 q2c
51 −1008c5
2 q2c51 + 32ηc4
2 q2c51 −3168c4
2 q2c51 −64ηc3
2 q2c51 + 2016c3
2 q2c51 −112ηc2
2 q2c51 + 2016c2
2 q2c51 +
64ηq2c51 + 32ηc2q2c
51 − 1008c2q2c
51 − 288q2c
51 + 432c5
1 + 26ηc62 c
41 + 1476c6
2 c41 − 304ηc5
2 c41 − 7056c5
2 c41 +
2724ηc42 c
41 + 21168c4
2 c41 − 1672ηc3
2 c41 − 3600c3
2 c41 + 3114ηc2
2 c41 − 15660c2
2 c41 + 138ηc6
2 q22c
41 + 6372c6
2 q22c
41 +
272ηc52 q
22c
41 + 10224c5
2 q22c
41 + 4292ηc4
2 q22c
41 + 15408c4
2 q22c
41 + 1496ηc3
2 q22c
41 + 8496c3
2 q22c
41 + 5018ηc2
2 q22c
41 + . . .
The Polynomial e(q2)
· · · − 25740c22 q
22c
41 + 1064ηq2
2c41 + 680ηc2q
22c
41 − 576c2q
22c
41 + 1368q2
2c41 + 616ηc4
1 − 760ηc2c41 − 2304c2c
41 +
60ηc62 q2c
41 + 1944c6
2 q2c41 + 32ηc5
2 q2c41 − 3168c5
2 q2c41 + 216ηc4
2 q2c41 − 11232c4
2 q2c41 + 176ηc3
2 q2c41 −
4896c32 q2c
41 − 228ηc2
2 q2c41 + 2808c2
2 q2c41 + 240ηq2c
41 + 80ηc2q2c
41 + 2880c2q2c
41 + 1296q2c
41 + 792c4
1 +
44ηc62 c
31 + 1584c6
2 c31 − 352ηc5
2 c31 − 5328c5
2 c31 − 1672ηc4
2 c31 − 3600c4
2 c31 − 1936ηc3
2 c31 + 17568c3
2 c31 −
1012ηc22 c
31 + 14544c2
2 c31 − 132ηc6
2 q22c
31 − 4752c6
2 q22c
31 + 416ηc5
2 q22c
31 + 3312c5
2 q22c
31 + 1496ηc4
2 q22c
31 +
8496c42 q
22c
31 + 1808ηc3
2 q22c
31 − 13536c3
2 q22c
31 + 1628ηc2
2 q22c
31 − 14832c2
2 q22c
31 + 176ηq2
2c31 + 944ηc2q
22c
31 −
144c2q22c
31 +720q2
2c31 −528ηc3
1 −880ηc2c31 −1872c2c
31 +88ηc6
2 q2c31 +3168c6
2 q2c31 −64ηc5
2 q2c31 +2016c5
2 q2c31 +
176ηc42 q2c
31 − 4896c4
2 q2c31 + 128ηc3
2 q2c31 − 4032c3
2 q2c31 − 616ηc2
2 q2c31 + 288c2
2 q2c31 + 352ηq2c
31 − 64ηc2q2c
31 +
2016c2q2c31 + 1440q2c
31 − 2160c3
1 + 41ηc62 c
21 + 936c6
2 c21 − 184ηc5
2 c21 + 720c5
2 c21 + 3114ηc4
2 c21 − 15660c4
2 c21 −
1012ηc32 c
21 + 14544c3
2 c21 + 3729ηc2
2 c21 − 15552c2
2 c21 + 177ηc6
2 q22c
21 + 4968c6
2 q22c
21 + 296ηc5
2 q22c
21 − 2736c5
2 q22c
21 +
5018ηc42 q
22c
21 −25740c4
2 q22c
21 + 1628ηc3
2 q22c
21 −14832c3
2 q22c
21 + 6041ηc2
2 q22c
21 −24768c2
2 q22c
21 + 1220ηq2
2c21 + . . .
The Polynomial e(q2)
· · · + 740ηc2q22c
21 + 4608c2q
22c
21 + 11844q2
2c21 + 676ηc2
1 − 460ηc2c21 + 2880c2c
21 + 54ηc6
2 q2c21 + 2160c6
2 q2c21 −
112ηc52 q2c
21 + 2016c5
2 q2c21 − 228ηc4
2 q2c21 + 2808c4
2 q2c21 − 616ηc3
2 q2c21 + 288c3
2 q2c21 − 1050ηc2
2 q2c21 −
10368c22 q2c
21 + 216ηq2c
21 − 280ηc2q2c
21 − 7488c2q2c
21 + 216q2c
21 + 6948c2
1 + 20ηc62 c1 + 288c6
2 c1 −
160ηc52 c1 + 1800c5
2 c1 − 760ηc42 c1 − 2304c4
2 c1 − 880ηc32 c1 − 1872c3
2 c1 − 460ηc22 c1 + 2880c2
2 c1 − 60ηc62 q
22c1 −
864c62 q
22c1 + 128ηc5
2 q22c1 − 792c5
2 q22c1 + 680ηc4
2 q22c1 − 576c4
2 q22c1 + 944ηc3
2 q22c1 − 144c3
2 q22c1 + 740ηc2
2 q22c1 +
4608c22 q
22c1 + 80ηq2
2c1 + 368ηc2q22c1 + 6120c2q
22c1 + 2016q2
2c1 − 240ηc1 − 400ηc2c1 − 5112c2c1 +
40ηc62 q2c1 + 576c6
2 q2c1 + 32ηc52 q2c1 − 1008c5
2 q2c1 + 80ηc42 q2c1 + 2880c4
2 q2c1 − 64ηc32 q2c1 + 2016c3
2 q2c1 −
280ηc22 q2c1 − 7488c2
2 q2c1 + 160ηq2c1 + 32ηc2q2c1 − 1008c2q2c1 + 4032q2c1 − 6048c1 + 4ηc62 + 36c6
2 −
96ηc52 + 432c5
2 + 616ηc42 + 792c4
2 − 528ηc32 − 2160c3
2 + 676ηc22 + 6948c2
2 + 36ηc62 q
22 + 324c6
2 q22 + 32ηc5
2 q22 −
144c52 q
22 + 1064ηc4
2 q22 + 1368c4
2 q22 + 176ηc3
2 q22 + 720c3
2 q22 + 1220ηc2
2 q22 + 11844c2
2 q22 + 272ηq2
2 + 80ηc2q22 +
2016c2q22 + 9792q2
2 + 144η − 240ηc2 − 6048c2 + 24ηc62 q2 + 216c6
2 q2 + 64ηc52 q2 − 288c5
2 q2 + 240ηc42 q2 +
1296c42 q2 + 352ηc3
2 q2 + 1440c32 q2 + 216ηc2
2 q2 + 216c22 q2 + 96ηq2 + 160ηc2q2 + 4032c2q2 + 3456q2 + 5184
The Bound
I Consider extreme cases for(c1, c2)
I Consider the limits η → 0and η →∞ 0 0.5 1
0
0.5
10 0.5 1
0
0.5
1
c1
c 2
This yields the following guess:
B(q2) =
(q2+3
4
)2, 0 < q2 < Q2,
q2(q2 + 1), Q2 ≤ q2 < 1.,
with Q2 = 115 (4√
10− 5).
The Bound
I Consider extreme cases for(c1, c2)
I Consider the limits η → 0and η →∞ 0 0.5 1
0
0.5
10 0.5 1
0
0.5
1
c1
c 2
This yields the following guess:
B(q2) =
(q2+3
4
)2, 0 < q2 < Q2,
q2(q2 + 1), Q2 ≤ q2 < 1.,
with Q2 = 115 (4√
10− 5).
Bound for the Convergence Rate
0.0 0.2 0.4 0.6 0.8 1.0Τ
0.2
0.4
0.6
0.8
1.0
1.2
1.4qTG
Two-grid convergence factor depending on τ forν = νpre + νpost = 2 + 2 smoothing steps
Experimental all-at-once analysis for 2D
I Set up the full matrix for the all-at-once approach
I Consider only the limiting cases for c1, c2, η
I qTG (τ) = max
116 (τ − 4)2, 1
4 |3τ − 2|√
9τ2 − 12τ + 8
http://www.risc.jku.at/people/vpillwei/sLFA/
Experimental all-at-once analysis for 2D
I Set up the full matrix for the all-at-once approach
I Consider only the limiting cases for c1, c2, η
I qTG (τ) = max
116 (τ − 4)2, 1
4 |3τ − 2|√
9τ2 − 12τ + 8
http://www.risc.jku.at/people/vpillwei/sLFA/
Experimental all-at-once analysis for 2D
I Set up the full matrix for the all-at-once approach
I Consider only the limiting cases for c1, c2, η
I qTG (τ) = max
116 (τ − 4)2, 1
4 |3τ − 2|√
9τ2 − 12τ + 8
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.2
0.4
0.6
0.8
1.0
1.2
1.4
http://www.risc.jku.at/people/vpillwei/sLFA/
Experimental all-at-once analysis for 2D
I Set up the full matrix for the all-at-once approach
I Consider only the limiting cases for c1, c2, η
I qTG (τ) = max
116 (τ − 4)2, 1
4 |3τ − 2|√
9τ2 − 12τ + 8
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.2
0.4
0.6
0.8
1.0
1.2
1.4
http://www.risc.jku.at/people/vpillwei/sLFA/
Chapter 0, Basic Concepts, Brenner+Scott, 2002
The finite element method provides a formalism for generatingdiscrete (finite) algorithms for approximating the solutions ofpartial differential equations. It should be thought of as a blackbox into which one puts the differential equation (boundary valueproblem) and out of which pops an algorithm for approximatingcorresponding solutions. Such a task could conceivably be doneautomatically by a computer, but it necessitates an amount ofmathematical skill that today still requires human involvement.The purpose of this book is to help people become adept atworking the magic of this black box. The book does not focus onhow to turn the resulting algorithms into computer codes, althoughthis is at present also a complicated task. The latter is, however, amore well-defined task than the former and thus potentially moreamenable to automation.