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Simplicial structures on train tracks. Fedor Duzhin, Nanyang Technological University, Singapore. Plan of the talk. Braid groups Crossed simplicial structure Free groups and simplicial group structure on free groups Combinatorial description of mapping classes

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simplicial structures on train tracks

Simplicial structures on train tracks

Fedor Duzhin, Nanyang Technological University, Singapore

plan of the talk
Plan of the talk
  • Braid groups
  • Crossed simplicial structure
  • Free groups and simplicial group structure on free groups
  • Combinatorial description of mapping classes
  • Simplicial structure on train tracks
braid groups
Braid groups
  • A braid is:
    • n descending strands
    • joins {(i/(n-1),0,1)} to {(i/(n-1),0,0)}
    • the strands do not intersect each other
    • considered up to isotopy in R3
    • multiplication from top to bottom
    • the unit braid

1 =


artin s presentation
Artin’s presentation

The braid group on n strands Bn has the following presentation:




braid groups on the sphere
Braid groups on the sphere

Given any space M, its n-th ordered configuration space is

Obviously, the symmetric group Sn acts on F(M,n) by permuting coordinates.

The braid group Bn is then

The pure braid group Bn is

Braid group on the sphere

Pure braid group on the sphere

symmetric groups
Symmetric groups

Symmetric group Sn consists of bijections of an n-element set to itself

Presentation with generators - transpositions


crossed simplicial structure
Crossed simplicial structure

The braid group is a crossed simplicial group, that is,

Homomorphism tothe permutation group



Simplicial identities

Crossed simplicial relation

crossed simplicial structure1
Crossed simplicial structure

Face-operators are given by deleting a strand:

crossed simplicial structure2
Crossed simplicial structure

Degeneracy-operators are given by doubling a strand:

important result
Important result

Jon Berrick, Fred Cohen, Yan-Loi Wong, Jie Wu:

There is a following exact sequence (actually, there are more)

Here the groups of Brunnian braids are

In other words, a braid is Brunnian if it becomes trivial after removing any its strand.

free group
Free group

Free group Fn:

Generators x0,x1,…,xn-1

No relations

Fn is the fundamental group of the n-punctured disk

AutFn is the group of automorphisms of Fn

Mapping class group consists of homotopy classes of self-homeomorphisms




free group as a simplicial group
Free group as a simplicial group

Also, Fn admits a simplicial group structure, that is,


(group homomorphisms)


(group homomorphisms)

Simplicial identities:

artin s representation
Artin’s representation

Artin’s representation is obtained from considering braids as mapping classes

The disk is made of rubber

Punctures are holes

The braid is made of wire

The disk is being pushed down along the braid

Theorem The braid group is isomorphic to the mapping class group of the punctured disk

artin s representation1
Artin’s representation

Braids and general automorphisms are applied to free words on the right

  • Theorem (Artin)
  • The Artin representation is faithful
  • The image of the Artin representation is the set of automorphisms given by



permutative action
Permutative action


Generally, the braid groups act on free groups so that

commute for any braid a

In particular, for a pure braid a, the permutation πa is identity, so we have

skeleton graphs
Skeleton graphs

Let S be an n-punctured disk (or, generally, a surface with n punctures and k boundary components)

A skeleton graph is homotopy equivalent to the entire surface. It consists of n closed edges encircling punctures and a tree. Also, there are some natural equivalence relations. For example, one can remove a vertex of valence 1 or 2

In order to give a combinatorial description to a mapping class (that is a self-homeomorphism of the surface S considered up to homotopy fixing the boundary of the disk pointwise and the set of punctures), one first defines a skeleton graph.

skeleton graphs1
Skeleton graphs

Given a homeomorphism f:S→S, the image of a skeleton graph is some other skeleton graph.


skeleton graphs2
Skeleton graphs

A map of a skeleton graph G to itself occurs as follows




skeleton graphs3
Skeleton graphs

Such a map induced on skeleton graphs is not a homeomorphism

For example, the following disk automorphism

bar means


induces graph map given by

Graph maps like this one are used in so called

train track algorithm (M. Bestvina, M. Handel)

simplicial structure on train tracks
Simplicial structure on train tracks

This is a current co-joint work with Jon Berrick and Jie Wu

Disclaimer: it’s not train tracks we construct simplicial structure on (train tracks will not even be defined in this talk)

We define a certain object called labelled skeleton graph. The set of labelled skeleton graphs is related to skeleton graph maps as

Skeleton graph maps

Skeleton graph mapping classes

Labelled skeleton graphs

Free group endomorphisms

labelled skeleton graph
Labelled skeleton graph

A labelled skeleton graphs looks like

Each edge is labelled by a free word

Closed edges are labelled by a permutation of the free generators

There are some equivalence relations

simplicial structure on labelled skeleton graphs

Thanks for your attention

Simplicial structure on labelled skeleton graphs

Face-operator kills a closed edge (and applies the free group face operator to all labels)



Degeneracy-operator inserts two new edges