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The cat’s cradle, stirring, and topological complexity Jean-Luc Thiffeault 1,2 Erwan Lanneau 3 Sarah Matz 1 1 Department of Mathematics University of Wisconsin – Madison 2 Institute for Mathematics and its Applications University of Minnesota – Twin Cities 3 Centre de Physique Th´ eorique, Universit´ e du Sud Toulon-Var and F´ ed´ eration de Recherches des Unit´ es de Math´ ematiques de Marseille, Luminy, France St. Olaf College, 3 November 2009 Based on a paper in Dynamical Systems Magazine, April 2009. 1/1
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The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Jun 27, 2020

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Page 1: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

The cat’s cradle, stirring,and topological complexity

Jean-Luc Thiffeault1,2 Erwan Lanneau3 Sarah Matz1

1Department of MathematicsUniversity of Wisconsin – Madison

2Institute for Mathematics and its ApplicationsUniversity of Minnesota – Twin Cities

3Centre de Physique Theorique, Universite du Sud Toulon-Var and Federation deRecherches des Unites de Mathematiques de Marseille, Luminy, France

St. Olaf College, 3 November 2009

Based on a paper in Dynamical Systems Magazine, April 2009.

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Page 2: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

The cat’s cradle

(Hands by Sarah Matz)

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Page 3: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

The cat’s cradle: How to

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Page 5: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Surfaces: Holes and handles

torus (genus = 1)

disk with 4 holes

2-torus (genus = 2)

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Page 6: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Mappings of surfaces

A continuous, invertible mapping φ and its action on two closedloops:

φ is called a homeomorphism.

In topology, only care about objects up to continuous deformation(homotopy) — equivalence classes of loops.

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Page 7: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Torus: Types of mappings

The topological types of mappings define the mapping class groupof a surface. For the torus, all that matters is what a mapping φdoes to loops.

=⇒ count how many times loops wrap around periodic directions.

Under the action of φ:

red:

(10

)7→(

21

)blue:

(01

)7→(

11

)

This is a linear transformation.

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Page 8: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Torus: Action on loops

Hence, the action on fundamental loops

red:

(10

)7→(

21

)blue:

(01

)7→(

11

)can be written as a matrix equation(

xy

)7→(

2 11 1

)(xy

)The matrix

M =

(2 11 1

)encapsulates everything we need to know (topologically) about themapping φ.

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Page 9: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Torus: Mapping class group

In mathematical language, we say that the the mapping classgroup of the torus is isomorphic to matrices:

MCG(torus) ' PSL2(Z)

PSL2(Z) the projective version of the group of invertibletwo-by-two matrices with determinant 1.

(A positive determinant guarantees orientability, and unitdeterminant means that the matrices can be inverted over theintegers.)

How can we understand the type of possible behavior of differentelements of the mapping class group?

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Page 10: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Classification of mappings

Consider a general element of MCG(torus), represented here as aninteger matrix

M =

(a bc d

)ad − bc = 1

One way to classify mapping classes is to examine the eigenvaluesof M.

Since det M = λ1λ2 = 1, M has two eigenvalues

λ1 = λ and λ2 = λ−1.

Without loss of generality, we assume |λ| ≥ 1. (λ could becomplex.)

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Page 11: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Characteristic polynomial

Recall the characteristic polynomial

p(x) = det(M − x I ) =

∣∣∣∣a− x bc d − x

∣∣∣∣ = (a− x)(d − x)− bc

= x2 − xd − ax + ad − bc

= x2 − (a + d)x + 1

= x2 − τ x + 1

where τ = a + d is the trace of M.

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Page 12: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Cayley–Hamilton theorem

The Cayley–Hamilton theorem states that any matrix is a zero ofits characteristic equation:

p(M) = M2 − τ M + I = 0 ⇐⇒ M2 = τ M − I

We can write M2 in terms of M!

Example:

M =

(2 11 1

)=⇒ τ = 2 + 1 = 3,

M2 =

(5 33 2

), 3M − I = 3

(2 11 1

)−(

1 00 1

)=

(5 33 2

)

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Page 13: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Classification, part 1

M2 = τ M − I

τ = a + d is the sum of two integers, so the smallest values |τ | cantake are 0 and ±1.

τ = 0: then M2 = −I , so M4 = (−I )2 = I .

M4 = I for τ = 0

τ = ±1: then M2 = ±M − I , so

M3 = M(M2) = M(±M − I ) = ±M2 −M = ±(±M − I )−M = ∓I

M6 = I for τ = ±1

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Page 14: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Finite-order mappings

Such mappings are called finite order: the loops eventually returnto their initial winding.

We can combine the previous two statements as

M12 = I |τ | < 2

(12 is the least common multiple of 4 and 6.)

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Page 15: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Classification, part 2

Now consider |τ | > 2.

We solve p(x) = 0, and find the largest eigenvalue

λ = sign τ × 12

(|τ |+

√τ2 − 4

)where λ is real and |λ| > 1 (i.e., strict).

The mapping classes with |λ| > 1 are called Anosov.

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Page 16: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Anosov maps

Representing the torus as a doubly-periodic interval [0, 1]× [0.1],first few iterates on a loop:

Asymptotically, the length of the loop is multiplied by |λ| eachtime ⇒ exponential growth.

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Page 17: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Arnold’s Cat map

For our example

M =

(2 11 1

)(shown on previous slide), we have

λ = 12 (3 +

√5) = ϕ2 ' 2.618 . . .

where ϕ is the Golden ratio.

For historical reasons, this particular linear map is called Arnold’scat map, after V. I. Arnold.

Anosov maps exhibit the purest form of chaotic behavior. Theycan only exist on a torus.

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Page 18: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Thurston–Nielsen classification theorem

More complex surfaces than the torus, such as the 2-torus or diskswith holes, require a powerful theorem:

Theorem (Thurston–Nielsen classification theorem)

Let φ by a homeomorphism of a surface M. The φ is isotopic to ψ,where ψ is one of three types:

1. finite-order

2. pseudo-Anosov

3. reducible

(For the experts: this is only for hyperbolic surfaces.)

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Page 19: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

pseudo-Anosovs

pseudo-Anosovs are much more complex (and interesting!) thanAnosovs, and are still the subject of active story. The “pseudo”comes from the presence of singularities.

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Page 20: The cat's cradle, stirring, and topological complexityjeanluc/talks/stolaf2009.pdfThe cat’s cradle, stirring, and topological complexity Jean-Luc Thi eault1;2 Erwan Lanneau3 Sarah

Summary

• Mappings of surfaces can be understood from their action oncurves.

• Physically, these arise for instance in fluid dynamics whenstirring with rods.

• In that case, the best stirring methods correspond topseudo-Anosov (pA) mappings.

• Many open questions about pAs:• Spectrum of values of λ;• How to construct for characteristic polynomial;• Connection to number theory?

• See article at arxiv.org/abs/0904.0778 for more details.

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