In mathematics, the free factor complex (sometimes also called the complex of free factors) is a free group counterpart of the notion of the curve complex of a finite type surface.
The free factor complex was originally introduced in a 1998 paper of
Gromov-hyperbolic
. The free factor complex plays a significant role in the study of large-scale geometry of .
Formal definition
For a free group a proper free factor of is a subgroup such that and that there exists a subgroup such that .
Let be an integer and let be the free group of rank . The free factor complex for is a simplicial complex where:
(1) The 0-cells are the conjugacy classes in of proper free factors of , that is
(2) For , a -simplex in is a collection of distinct 0-cells such that there exist free factors of such that for , and that . [The assumption that these 0-cells are distinct implies that for ]. In particular, a 1-cell is a collection of two distinct 0-cells where are proper free factors of such that .
For the above definition produces a complex with no -cells of dimension . Therefore, is defined slightly differently. One still defines to be the set of conjugacy classes of proper free factors of ; (such free factors are necessarily infinite cyclic). Two distinct 0-simplices determine a 1-simplex in if and only if there exists a free basis of such that .
The complex has no -cells of dimension .
For the 1-skeleton is called the free factor graph for .
Main properties
For every integer the complex is connected, locally infinite, and has dimension . The complex is connected, locally infinite, and has dimension 1.
For , the graph is isomorphic to the
Farey graph
.
There is a natural
action
of on by simplicial automorphisms. For a k-simplex and one has .
For every integer , the free factor graph , equipped with the simplicial metric (where every edge has length 1), is a connected graph of infinite diameter.[2][3]
For every integer , the free factor graph , equipped with the simplicial metric, is
Gromov-hyperbolic. This result was originally established by Mladen Bestvina and Mark Feighn;[4] see also [5][6]
for subsequent alternative proofs.
An element acts as a loxodromic isometry of if and only if is fully irreducible.[4]
There exists a coarsely Lipschitz coarsely -equivariant coarsely surjective map , where is the
Gromov-hyperbolic, as was proved by Handel and Mosher.[7]
Similarly, there exists a natural coarsely Lipschitz coarsely -equivariant coarsely surjective map , where is the (volume-ones normalized) Culler–Vogtmann Outer space, equipped with the symmetric Lipschitz metric. The map takes a geodesic path in to a path in contained in a uniform Hausdorff neighborhood of the geodesic with the same endpoints.[4]
The hyperbolic boundary of the free factor graph can be identified with the set of equivalence classes of "arational" -trees in the boundary of the Outer space .[8]
The free factor complex is a key tool in studying the behavior of random walks on and in identifying the Poisson boundary of .[9]
Other models
There are several other models which produce graphs coarsely -equivariantly quasi-isometric to . These models include:
The graph whose vertex set is and where two distinct vertices are adjacent if and only if there exists a free product decomposition such that and .
The free bases graph whose vertex set is the set of -conjugacy classes of free bases of , and where two vertices are adjacent if and only if there exist free bases of such that and .[5]