Dessin d’Enfants Examples due to Magot and Zvonkin Moduli Spaces From Klein’s Platonic Solids to Kepler’s Archimedean Solids: Elliptic Curves and Dessins d’Enfants Part II Edray Herber Goins Department of Mathematics Purdue University September 7, 2012 Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
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Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
From Klein’s Platonic Solidsto Kepler’s Archimedean Solids:
Elliptic Curves and Dessins d’Enfants
Part II
Edray Herber Goins
Department of MathematicsPurdue University
September 7, 2012
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Abstract
In 1884, Felix Klein wrote his influential book, “Lectures on theIcosahedron,” where he explained how to express the roots of anyquintic polynomial in terms of elliptic modular functions. His idea wasto relate rotations of the icosahedron with the automorphism groupof 5-torsion points on a suitable elliptic curve. In fact, he created atheory which related rotations of each of the five regular solids (thetetrahedron, cube, octahedron, icosahedron, and dodecahedron) withthe automorphism groups of 3-, 4-, and 5-torsion points.
Using modern language, the functions which relate the rotations withelliptic curves are Belyı maps. In 1984, Alexander Grothendieckintroduced the concept of a Dessin d’Enfant in order to understandGalois groups via such maps. We will complete a circle of ideas byreviewing Klein’s theory with an emphasis on the octahedron;explaining how to realize the five regular solids (the Platonic solids)as well as the thirteen semi-regular solids (the Archimedean solids) asDessins d’Enfant; and discussing how the corresponding Belyı mapsrelate to moduli spaces of elliptic curves.
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Outline of Talk
1 Dessin d’EnfantsPlatonic SolidsKlein’s Theory of CovariantsBelyı’s Theorem
2 Examples due to Magot and ZvonkinRotation Group Dn
Rotation Groups A4 and S4
Rotation Group A5
3 Moduli SpacesX (3), X (4), and X (5)X0(2), X (2), and X (2, 4)Torsion on Elliptic Curves
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Platonic SolidsKlein’s Theory of CovariantsBelyı’s Theorem
Recap
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Platonic SolidsKlein’s Theory of CovariantsBelyı’s Theorem
What is a Platonic Solid?
A regular, convex polyhedron is one of the collections of vertices
V =
{(z1 : z0) ∈ P1(C)
∣∣∣∣ δ(z1, z0) = 0
}in terms of the homogeneous polynomials
δ(z1, z0) =
z1n + z0
n for the regular polygon,
z1(z1
3 − z03)
for the tetrahedron,
z1 z0(z1
4 − z04)
for the octahedron,
z18 + 14 z1
4 z04 + z0
8 for the cube,
z1 z0(z1
10 − 11 z15 z0
5 − z010)
for the icosahedron,
z120 + 228 z1
15 z05 + 494 z1
10 z010
− 228 z15 z0
15 + z020
for the dodecahedron.
We embed V ↪→ S2(R) into the unit sphere via stereographic projection.
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Platonic SolidsKlein’s Theory of CovariantsBelyı’s Theorem
http://mathworld.wolfram.com/RegularPolygon.html
http://en.wikipedia.org/wiki/Platonic_solids
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Platonic SolidsKlein’s Theory of CovariantsBelyı’s Theorem
Rigid Rotations of the Platonic Solids
Recall the action ◦ : PSL2(C)× P1(C)→ P1(C). We seek Galois extensionsQ(ζn, z)/Q(ζn, j) for rational j(z).
Zn =⟨r∣∣ rn = 1
⟩and Dn =
⟨r , s
∣∣ s2 = rn = (s r)2 = 1⟩
are the rigidrotations of the regular convex polygons, with
r(z) = ζn z, s(z) =1
z, and j(z) = 1728 zn or
6912 zn
(zn + 1)2.
A4 =⟨r , s
∣∣ s2 = r 3 = (s r)3 = 1⟩' PSL2(F3) are the rigid rotations of
the tetrahedron, with
r(z) = ζ3 z, s(z) =1 − z
2 z + 1, and j(z) = −
27 (8 z3 + 1)3
z3 (z3 − 1)3.
S4 =⟨r , s
∣∣ s2 = r 3 = (s r)4 = 1⟩' PGL2(F3) ' PSL2(Z/4Z) are the
rigid rotations of the octahedron and the cube, with
r(z) =ζ4 + z
ζ4 − z, s(z) =
1 − z
1 + z, and j(z) =
16 (z8 + 14 z4 + 1)3
z4 (z4 − 1)4.
A5 =⟨r , s
∣∣ s2 = r 3 = (s r)5 = 1⟩' PSL2(F4) ' PSL2(F5) are the rigid
rotations of the icosahedron and the dodecahedron, with
r(z) =
(ζ5 + ζ5
4) ζ5 − z(ζ5 + ζ5
4)z + ζ5
, s(z) =
(ζ5 + ζ5
4) − z(ζ5 + ζ5
4)z + 1
, j(z) =(z20 + 228 z15 + 494 z10 − 228 z5 + 1)3
z5 (z10 − 11 z5 − 1)5.
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Platonic SolidsKlein’s Theory of CovariantsBelyı’s Theorem
Klein’s Theory of Covariants
Theorem (Felix Klein, 1884)
The rational function
j(z) =
−27
(8 z3 + 1
)3z3 (z3 − 1)3
for n = 3,
16
(z8 + 14 z4 + 1
)3z4 (z4 − 1)4
for n = 4,(z20 + 228 z15 + 494 z10 − 228 z5 + 1
)3z5 (z10 − 11 z5 − 1)5
for n = 5;
is invariant under the group G =⟨r , s
∣∣ s2 = r 3 = (s r)n = 1⟩
expressed interms of the generators
r(z) =
ζ3 z for n = 3,
ζ4 + z
ζ4 − zfor n = 4,(
ζ5 + ζ54)ζ5 − z(
ζ5 + ζ54)z + ζ5
for n = 5;
s(z) =
1 − z
2 z + 1for n = 3,
1 − z
1 + zfor n = 4,(
ζ5 + ζ54)− z(
ζ5 + ζ54)z + 1
for n = 5.
In particular, Gal(Q(ζn, z)/Q(ζn, j)
)' G ' PSL2(Z/nZ).
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Platonic SolidsKlein’s Theory of CovariantsBelyı’s Theorem
Elliptic Curves Associated to Principal Polynomials
Theorem (Felix Klein, 1884; G, 1999)
Set n = 3, 4, 5. Let q(x) = xn + Axn−3 + · · ·+ B x + C be over K = Q(ζn)with splitting field L, and assume that Gal(L/K) ' PSL2(Z/nZ). Then thereexists j ∈ K such that LK = K(E [n]x) is the field generated by thex-coordinates of the n-torsion of an elliptic curve E with invariant j .
If we define the rational functions
λ(z) =
8 z3 + 1
z (z3 − 1)for n = 3,
(1 + ζ4)[z2 − (1 + ζ4) z − ζ4
] [z2 − (1− ζ4) z + ζ4
] [z2 + (1 + ζ4) z − ζ4
]z (z4 − 1)
for n = 4,
[z2 + 1
]2 [z2 + 2
(ζ5 + ζ5
4)z − 1
]2 [z2 + 2
(ζ5
2 + ζ53)z − 1
]2z(z10 − 11 z5 − 1
) + 3 for n = 5;
then we have the polynomials
q(x) =∏ν
[x −
1
λ(ζnν z
) ] =
x3 +1
jfor n = 3,
x4 +32
jx +
4
jfor n = 4,
x5 −40
jx2 −
5
jx −
1
jfor n = 5.
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Platonic SolidsKlein’s Theory of CovariantsBelyı’s Theorem
Motivating Question
Let X be a compact Riemann surface. Fix a function φ : X → P1(C). For eachz in the inverse image of the thrice punctured sphere
φ−1
(P1(C)− {0, 1, ∞}
)⊆ X
form the elliptic curve
E : y 2 = x3 +3
φ(z)− 1x +
2
φ(z)− 1where j(E) =
1728
φ(z).
What are the properties of this elliptic curve?
Which types of functions φ : X → P1(C) are allowed?
If φ(z) has “lots of symmetries,” how does this translate into properties ofthe torsion elements K(E [n]x)?
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Platonic SolidsKlein’s Theory of CovariantsBelyı’s Theorem
A compact, connected Riemann surface X can be defined by a polynomialequation
∑i,j aij z
i w j = 0 where the coefficients aij are not transcendental if
and only if there exists a rational function φ : X → P1(C) which has at mostthree critical values
{0, 1, ∞
}.
“This discovery, which is technically so simple, made a very strong impressionon me, and it represents a decisive turning point in the course of myreflections, a shift in particular of my centre of interest in mathematics, whichsuddenly found itself strongly focused. I do not believe that a mathematicalfact has ever struck me quite so strongly as this one, nor had a comparablepsychological impact.
– Alexander Grothendieck, Esquisse d’un Programme (1984)
Definition
A rational function φ : X → P1(C) which has at most three critical values{0, 1, ∞
}is called a Belyı map.
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Platonic SolidsKlein’s Theory of CovariantsBelyı’s Theorem
Dessins d’Enfant
Fix a Belyı map φ : X → P1(C). Denote the preimages
B = φ−1({0})
W = φ−1({1})
E = φ−1([0, 1]
)X
φ−−−−−→ P1(C)y y y y{black
vertices
} {white
vertices
} {edges
}R3
The bipartite graph Γφ =(V ,E
)with vertices V = B ∪W and edges E is
called Dessin d’Enfant. We embed the graph on X in 3-dimensions.
I do not believe that a mathematical fact has ever struck me quite so stronglyas this one, nor had a comparable psychological impact. This is surely becauseof the very familiar, non-technical nature of the objects considered, of whichany child’s drawing scrawled on a bit of paper (at least if the drawing is madewithout lifting the pencil) gives a perfectly explicit example. To such a dessinwe find associated subtle arithmetic invariants, which are completely turnedtopsy-turvy as soon as we add one more stroke.
– Alexander Grothendieck, Esquisse d’un Programme (1984)
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Platonic SolidsKlein’s Theory of CovariantsBelyı’s Theorem
Must we work with
Platonic Solids?
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Rotation Group DnRotation Groups A4 and S4Rotation Group A5
Platonic Solids, Archimedean Solids, and Catalan Solids
Definition
A Platonic solid is a regular, convex polyhedron. They are named afterPlato (424 BC – 348 BC). Aside from the regular polygons, there are fivesuch solids.
An Archimedean solid is a convex polyhedron that has a similararrangement of nonintersecting regular convex polygons of two or moredifferent types arranged in the same way about each vertex with all sidesthe same length. Discovered by Johannes Kepler (1571 – 1630) in 1620,they are named after Archimedes (287 BC – 212 BC). Aside from theprisms and antiprisms, there are thirteen such solids.
A Catalan solid is a dual polyhedron to an Archimedean solid. They arenamed after Eugene Catalan (1814 – 1894) who discovered them in 1865.Aside from the bipyramids and trapezohedra, there are thirteen such solids.
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Rotation Group DnRotation Groups A4 and S4Rotation Group A5
Platonic Solid Archimedean Solid Catalan Solid Rotation Group
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids
Dessin d’EnfantsExamples due to Magot and Zvonkin
Moduli Spaces
Rotation Group DnRotation Groups A4 and S4Rotation Group A5
Proposition (Wushi Goldring, 2012)
Let ψ(w) be a rational function. The composition ψ ◦ φ is also a Belyı map forevery Belyı map φ : X → P1(C) if and only if ψ is a Belyı map which sends theset{
0, 1, ∞}
to itself.
Proposition (Nicolas Magot and Alexander Zvonkin, 2000)
The following ψ are Belyı maps which send the set{
0, 1, ∞}
to itself.
ψ(w) =
−(w − 1)2/(4w) is a rectification,
(4w − 1)3/(27w) is a truncation,
1/w is a birectification,
(4− w)3/(27w 2) is a bitruncation,
4 (w 2 − w + 1)3/(27w 2 (w − 1)2
)is a rhombitruncation,
(w + 1)4/(16w (w − 1)2
)is a rhombification,
7496192 (w + θ)5
25 (3 + 8 θ)w(88w − (57 θ + 64)
)3 is a snubification,
where θ2 − (7/64) θ + 1 = 0.
Number Theory Seminar From Klein’s Platonic Solids to Kepler’s Archimedean Solids