# Outer space (mathematics)

In the mathematical subject of geometric group theory, the Culler–Vogtmann Outer space or just Outer space of a free group Fn is a topological space consisting of the so-called "marked metric graph structures" of volume 1 on Fn. The Outer space, denoted Xn or CVn, comes equipped with a natural action of the group of outer automorphisms Out(Fn) of Fn. The Outer space was introduced in a 1986 paper,[1] of Marc Culler and Karen Vogtmann and it serves as a free group analog of the Teichmüller space of a hyperbolic surface. Outer space is used to study homology and cohomology groups of Out(Fn) and to obtain information about algebraic, geometric and dynamical properties of Out(Fn), of its subgroups and individual outer automorphisms of Fn. The space Xn can also be thought of as the set of Fn-equivariant isometry types of minimal free discrete isometric actions of Fn on Fn on R-trees T such that the quotient metric graph T/Fn has volume 1.

## History

The Outer space ${\displaystyle X_{n}}$  was introduced in a 1986 paper,[1] of Marc Culler and Karen Vogtmann, inspired by analogy with the Teichmüller space of a hyperbolic surface. They showed that the natural action of ${\displaystyle \operatorname {Out} (F_{n})}$  on ${\displaystyle X_{n}}$  is properly discontinuous, and that ${\displaystyle X_{n}}$  is contractible.

In the same paper Culler and Vogtmann constructed an embedding, via the translation length functions discussed below, of ${\displaystyle X_{n}}$  into the infinite dimensional projective space ${\displaystyle \mathbb {P} ^{\mathcal {C}}=\mathbb {R} ^{\mathcal {C}}-\{0\}/\mathbb {R} _{>0}}$ , where ${\displaystyle {\mathcal {C}}}$  is the set of nontrivial conjugacy classes of elements of ${\displaystyle F_{n}}$ . They also proved that the closure ${\displaystyle {\overline {X}}_{n}}$  of ${\displaystyle X_{n}}$  in ${\displaystyle \mathbb {P} ^{\mathcal {C}}}$  is compact.

Later a combination of the results of Cohen and Lustig[2] and of Bestvina and Feighn[3] identified (see Section 1.3 of[4])the space ${\displaystyle {\overline {X}}_{n}}$  with the space ${\displaystyle {\overline {CV}}_{n}}$  of projective classes of "very small" minimal isometric actions of ${\displaystyle F_{n}}$  on ${\displaystyle \mathbb {R} }$ -trees.

## Formal definition

### Marked metric graphs

Let n ≥ 2. For the free group Fn fix a "rose" Rn, that is a wedge, of n circles wedged at a vertex v, and fix an isomorphism between Fn and the fundamental group π1(Rn, v) of Rn. From this point on we identify Fn and π1(Rn, v) via this isomorphism.

A marking on Fn consists of a homotopy equivalence f : Rn → Γ where Γ is a finite connected graph without degree-one and degree-two vertices. Up to a (free) homotopy, f is uniquely determined by the isomorphism f# : π1(Rn) → π1(Γ), that is by an isomorphism Fnπ1(Γ).

A metric graph is a finite connected graph ${\displaystyle \gamma }$  together with the assignment to every topological edge e of Γ of a positive real number L(e) > 0 called the length of e. The volume of a metric graph is the sum of the lengths of its topological edges.

A marked metric graph structure on Fn consists of a marking f : Rn → Γ together with a metric graph structure L on Γ.

Two marked metric graph structures f1 : Rn → Γ1 and f2 : Rn → Γ2 are equivalent if there exists an isometry θ : Γ1 → Γ2 such that, up to free homotopy, we have θ o f1 = f2.

The Outer space Xn consists of equivalence classes of all the volume-one marked metric graph structures on Fn.

### Weak topology on the Outer space

#### Open simplices

Let f : Rn → Γ where Γ is a marking and let k be the number of topological edges in Γ. We order the edges of Γ as e1,..., ek. Let

${\displaystyle \Delta _{k}=\left\{(x_{1},\dots ,x_{k})\in \mathbb {R} ^{k}\,{\Big |}\,\sum _{i=1}^{k}x_{i}=1,x_{i}>0{\text{ for }}i=1,\dots ,k\right\}}$

be the standard (k − 1)-dimensional open simplex in Rk.

Given f, there is a natural map j : ΔkXn, where for x = (x1,..., xk) ∈ Δk, the point j(x) of Xn is given by the marking f together with the metric graph structure L on Γ such that L(ei) = xi for i = 1,...,k.

One can show that j is in fact an injective map, that is, distinct points of Δk correspond to non-equivalent marked metric graph structures on Fn.

The set jk) is called open simplex in Xn corresponding to f and is denoted S(f). By construction, Xn is the union of open simplices corresponding to all markings on Fn. Note that two open simplices in Xn either are disjoint or coincide.

#### Closed simplices

Let f : Rn → Γ where Γ is a marking and let k be the number of topological edges in Γ. As before, we order the edges of Γ as e1,..., ek. Define Δk′ ⊆ Rk as the set of all x = (x1,..., xk) ∈ Rk, such that ${\displaystyle \sum _{i=1}^{k}x_{i}=1}$ , such that each xi ≥ 0 and such that the set of all edges ei in ${\displaystyle \Gamma }$  with xi = 0 is a subforest in Γ.

The map j : ΔkXn extends to a map h : Δk′ → Xn as follows. For x in Δk put h(x) = j(x). For x ∈ Δk′ − Δk the point h(x) of Xn is obtained by taking the marking f, contracting all edges ei of ${\displaystyle \Gamma }$  with xi = 0 to obtain a new marking f1 : Rn → Γ1 and then assigning to each surviving edge ei of Γ1 length xi > 0.

It can be shown that for every marking f the map h : Δk′ → Xn is still injective. The image of h is called the closed simplex in Xn corresponding to f and is denoted by S′(f). Every point in Xn belongs to only finitely many closed simplices and a point of Xn represented by a marking f : Rn → Γ where the graph Γ is tri-valent belongs to a unique closed simplex in Xn, namely S′(f).

The weak topology on the Outer space Xn is defined by saying that a subset C of Xn is closed if and only if for every marking f : Rn → Γ the set h−1(C) is closed in Δk′. In particular, the map h : Δk′ → Xn is a topological embedding.

### Points of Outer space as actions on trees

Let x be a point in Xn given by a marking f : Rn → Γ with a volume-one metric graph structure L on Γ. Let T be the universal cover of Γ. Thus T is a simply connected graph, that is T is a topological tree. We can also lift the metric structure L to T by giving every edge of T the same length as the length of its image in Γ. This turns T into a metric space (T,d) which is a real tree. The fundamental group π1(Γ) acts on T by covering transformations which are also isometries of (T,d), with the quotient space T/π1(Γ) = Γ. Since the induced homomorphism f# is an isomorphism between Fn = π1(Rn) and π1(Γ), we also obtain an isometric action of Fn on T with T/Fn = Γ. This action is free and discrete. Since Γ is a finite connected graph with no degree-one vertices, this action is also minimal, meaning that T has no proper Fn-invariant subtrees.

Moreover, every minimal free and discrete isometric action of Fn on a real tree with the quotient being a metric graph of volume one arises in this fashion from some point x of Xn. This defines a bijective correspondence between Xn and the set of equivalence classes of minimal free and discrete isometric actions of Fn on a real trees with volume-one quotients. Here two such actions of Fn on real trees T1 and T2 are equivalent if there exists an Fn-equivariant isometry between T1 and T2.

#### Length functions

Give an action of Fn on a real tree T as above, one can define the translation length function associate with this action:

${\displaystyle \ell _{T}:F_{n}\to \mathbb {R} ,\quad \ell _{T}(g)=\min _{t\in T}d(t,gt),\quad {\text{ for }}g\in F_{n}.}$

For g ≠ 1 there is a (unique) isometrically embedded copy of R in T, called the axis of g, such that g acts on this axis by a translation of magnitude ${\displaystyle \ell _{T}(g)>0}$ . For this reason ${\displaystyle \ell _{T}(g)}$  is called the translation length of g. For any g, u in Fn we have ${\displaystyle \ell _{T}(ugu^{-1})=\ell _{T}(g)}$ , that is the function ${\displaystyle \ell _{T}}$  is constant on each conjugacy class in G.

In the marked metric graph model of Outer space translation length functions can be interpreted as follows. Let T in Xn be represented by a marking f : Rn → Γ with a volume-one metric graph structure L on Γ. Let gFn = π1(Rn). First push g forward via f# to get a closed loop in Γ and then tighten this loop to an immersed circuit in Γ. The L-length of this circuit is the translation length ${\displaystyle \ell _{T}(g)}$  of g.

A basic general fact from the theory of group actions on real trees says that a point of the Outer space is uniquely determined by its translation length function. Namely if two trees with minimal free isometric actions of Fn define equal translation length functions on Fn then the two trees are Fn-equivariantly isometric. Hence the map ${\displaystyle T\mapsto \ell _{T}}$  from Xn to the set of R-valued functions on Fn is injective.

One defines the length function topology or axes topology on Xn as follows. For every T in Xn, every finite subset K of Fn and every ε > 0 let

${\displaystyle V_{T}(K,\epsilon )=\{T'\in X_{n}:|\ell _{T}(g)-\ell _{T'}(g)|<\epsilon {\text{ for every }}g\in K\}.}$

In the length function topology for every T in Xn a basis of neighborhoods of T in Xn is given by the family VT(K, ε) where K is a finite subset of Fn and where ε > 0.

Convergence of sequences in the length function topology can be characterized as follows. For T in Xn and a sequence Ti in Xn we have ${\displaystyle \lim _{i\to \infty }T_{i}=T}$  if and only if for every g in Fn we have ${\displaystyle \lim _{i\to \infty }\ell _{T_{i}}(g)=\ell _{T}(g)}$ .

### Gromov topology

Another topology on ${\displaystyle X_{n}}$  is the so-called Gromov topology or the equivariant Gromov–Hausdorff convergence topology, which provides a version of Gromov–Hausdorff convergence adapted to the setting of an isometric group action.

When defining the Gromov topology, one should think of points of ${\displaystyle X_{n}}$  as actions of ${\displaystyle F_{n}}$  on ${\displaystyle \mathbb {R} }$ -trees. Informally, given a tree ${\displaystyle T\in X_{n}}$ , another tree ${\displaystyle T'\in X_{n}}$  is "close" to ${\displaystyle T}$  in the Gromov topology, if for some large finite subtrees of ${\displaystyle Y\subseteq T,Y'\subseteq T'\in X_{n}}$  and a large finite subset ${\displaystyle B\subseteq F_{n}}$  there exists an "almost isometry" between ${\displaystyle Y'}$  and ${\displaystyle Y}$  with respect to which the (partial) actions of ${\displaystyle B}$  on ${\displaystyle Y'}$  and ${\displaystyle Y}$  almost agree. For the formal definition of the Gromov topology see.[5]

#### Coincidence of the weak, the length function and Gromov topologies

An important basic result states that the Gromov topology, the weak topology and the length function topology on Xn coincide.[6]

## Action of Out(Fn) on Outer space

The group Out(Fn) admits a natural right action by homeomorphisms on Xn.

First we define the action of the automorphism group Aut(Fn) on Xn. Let α ∈ Aut(Fn) be an automorphism of Fn. Let x be a point of Xn given by a marking f : Rn → Γ with a volume-one metric graph structure L on Γ. Let τ : RnRn be a homotopy equivalence whose induced homomorphism at the fundamental group level is the automorphism α of Fn = π1(Rn). The element of Xn is given by the marking f o τ : Rn → Γ with the metric structure L on Γ. That is, to get x α from x we simply precompose the marking defining x with τ.

In the real tree model this action can be described as follows. Let T in Xn be a real tree with a minimal free and discrete co-volume-one isometric action of Fn. Let α ∈ Aut(Fn). As a metric space, is equal to T. The action of Fn is twisted by α. Namely, for any t in T and g in Fn we have:

${\displaystyle g{\underset {T\alpha }{\cdot }}t=\alpha (g){\underset {T}{\cdot }}t.}$

At the level of translation length functions the tree is given as:

${\displaystyle \ell _{T\alpha }(g)=\ell _{T}(\alpha (g))\quad {\text{ for }}g\in F_{n}.}$

One then checks that for the above action of Aut(Fn) on Outer space Xn the subgroup of inner automorphisms Inn(Fn) is contained in the kernel of this action, that is every inner automorphism acts trivially on Xn. It follows that the action of Aut(Fn) on Xn quotients through to an action of Out(Fn) = Aut(Fn)/Inn(Fn) on Xn. namely, if φ ∈ Out(Fn) is an outer automorphism of Fn and if α in Aut(Fn) is an actual automorphism representing φ then for any x in Xn we have = .

The right action of Out(Fn) on Xn can be turned into a left action via a standard conversion procedure. Namely, for φ ∈ Out(Fn) and x in Xn set

φ x = x φ−1.

This left action of Out(Fn) on Xn is also sometimes considered in the literature although most sources work with the right action.

### Moduli space

The quotient space Mn = Xn/Out(Fn) is the moduli space which consists of isometry types of finite connected graphs Γ without degree-one and degree-two vertices, with fundamental groups isomorphic to Fn (that is, with the first Betti number equal to n) equipped with volume-one metric structures. The quotient topology on Mn is the same as that given by the Gromov–Hausdorff distance between metric graphs representing points of Mn. The moduli space Mn is not compact and the "cusps" in Mn arise from decreasing towards zero lengths of edges for homotopically nontrivial subgraphs (e.g. an essential circuit) of a metric graph Γ.

## Basic properties and facts about Outer space

• Outer space Xn is contractible and the action of Out(Fn) on Xn is properly discontinuous, as was proved by Culler and Vogtmann in their original 1986 paper[1] where Outer space was introduced.
• The space Xn has topological dimension 3n − 4. The reason is that if Γ is a finite connected graph without degree-one and degree-two vertices with fundamental group isomorphic to Fn, then Γ has at most 3n − 3 edges and it has exactly 3n − 3 edges when Γ is trivalent. Hence the top-dimensional open simplex in Xn has dimension 3n − 4.
• Outer space Xn contains a specific deformation retract Kn of Xn, called the spine of Outer space. The spine Kn has dimension 2n − 3, is Out(Fn)-invariant and has compact quotient under the action of Out(Fn).

## Unprojectivized Outer space

The unprojectivized Outer space ${\displaystyle cv_{n}}$  consists of equivalence classes of all marked metric graph structures on Fn where the volume of the metric graph in the marking is allowed to be any positive real number. The space ${\displaystyle cv_{n}}$  can also be thought of as the set of all free minimal discrete isometric actions of Fn on R-trees, considered up to Fn-equivariant isometry. The unprojectivized Outer space inherits the same structures that ${\displaystyle X_{n}}$  has, including the coincidence of the three topologies (Gromov, axes, weak), and an ${\displaystyle \operatorname {Out} (F_{n})}$ -action. In addition, there is a natural action of ${\displaystyle \mathbb {R} _{>0}}$  on ${\displaystyle cv_{n}}$  by scalar multiplication.

Topologically, ${\displaystyle cv_{n}}$  is homeomorphic to ${\displaystyle X_{n}\times (0,\infty )}$ . In particular, ${\displaystyle cv_{n}}$  is also contractible.

## Projectivized Outer space

The projectivized Outer space is the quotient space ${\displaystyle CV_{n}:=cv_{n}/\mathbb {R} _{>0}}$  under the action of ${\displaystyle \mathbb {R} _{>0}}$  on ${\displaystyle cv_{n}}$  by scalar multiplication. The space ${\displaystyle CV_{n}}$  is equipped with the quotient topology. For a tree ${\displaystyle T\in cv_{n}}$  its projective equivalence class is denoted ${\displaystyle [T]=\{cT|c>0\}\subseteq cv_{n}}$ . The action of ${\displaystyle \operatorname {Out} (F_{n})}$  on ${\displaystyle cv_{n}}$  naturally quotients through to the action of ${\displaystyle \operatorname {Out} (F_{n})}$  on ${\displaystyle CV_{n}}$ . Namely, for ${\displaystyle \phi \in \operatorname {Out} (F_{n})}$  and ${\displaystyle T\in cv_{n}}$  put ${\displaystyle [T]\phi :=[T\phi ]}$ .

A key observation is that the map ${\displaystyle X_{n}\to CV_{n},T\mapsto [T]}$  is an ${\displaystyle \operatorname {Out} (F_{n})}$ -equivariant homeomorphism. For this reason the spaces ${\displaystyle X_{n}}$  and ${\displaystyle CV_{n}}$  are often identified.

## Lipschitz distance

The Lipschitz distance,[7] named for Rudolf Lipschitz, for Outer space corresponds to the Thurston metric in Teichmüller space. For two points ${\displaystyle x}$ ,${\displaystyle y}$  in Xn the (right) Lipschitz distance ${\displaystyle d_{R}}$  is defined as the (natural) logarithm of the maximally stretched closed path from ${\displaystyle x}$  to ${\displaystyle y}$ :

${\displaystyle \Lambda _{R}(x,y):=\sup _{\gamma \in F_{n}\setminus \{1\}}{\frac {\ell _{y}(\gamma )}{\ell _{x}(\gamma )}}}$  and ${\displaystyle d_{R}(x,y):=\log \Lambda _{R}(x,y)}$

This is an asymmetric metric (also sometimes called a quasimetric), i.e. it only fails symmetry ${\displaystyle d_{R}(x,y)=d_{R}(y,x)}$ . The symmetric Lipschitz metric normally denotes:

${\displaystyle d(x,y):=d_{R}(x,y)+d_{R}(y,x)}$

The supremum ${\displaystyle \Lambda _{R}(x,y)}$  is always obtained and can be calculated by a finite set the so called candidates of ${\displaystyle x}$ .

${\displaystyle \Lambda _{R}(x,y)=\max _{\gamma \in cand(x)}{\frac {\ell _{y}(\gamma )}{\ell _{x}(\gamma )}}}$

A simple loop, a figure of eight, and a barbell

Where ${\displaystyle cand(x)}$  is the finite set of conjugacy classes in Fn which correspond to embeddings of a simple loop, a figure of eight, or a barbell into ${\displaystyle x}$  via the marking.

The stretching factor also equals the minimal Lipschitz constant of a homotopy equivalence carrying over the marking, i.e.

${\displaystyle \Lambda _{R}(x,y)=\min _{h\in H(x,y)}Lip(h)}$

Where ${\displaystyle H(x,y)}$  are the continuous functions ${\displaystyle h:x\to y}$  such that for the marking ${\displaystyle f_{x}}$  on ${\displaystyle x}$  the marking ${\displaystyle h\circ f_{x}}$  is freely homotopic to the marking ${\displaystyle f_{y}}$  on ${\displaystyle y}$ .

The induced topology is the same as the weak topology and the isometry group is ${\displaystyle \operatorname {Out} (F_{n})}$  for both, the symmetric and asymmetric Lipschitz distance.[8]

## Applications and generalizations

• The closure ${\displaystyle {\overline {cv}}_{n}}$  of ${\displaystyle cv_{n}}$  in the length function topology is known to consists of (Fn-equivariant isometry classes) of all very small minimal isometric actions of Fn on R-trees.[9] Here the closure is taken in the space of all minimal isometric "irreducible" actions of ${\displaystyle F_{n}}$  on ${\displaystyle \mathbb {R} }$ -trees, considered up-to equivariant isometry. It is known that the Gromov topology and the axes topology on the space of irreducible actions coincide,[5] so the closure can be understood in either sense. The projectivization of ${\displaystyle {\overline {cv}}_{n}}$  with respect to multiplication by positive scalars gives the space ${\displaystyle {\overline {CV}}_{n}}$  which is the length function compactification of ${\displaystyle CV_{n}}$  and of ${\displaystyle X_{n}}$ , analogous to Thurston's compactification of the Teichmüller space.
• Analogs and generalizations of the Outer space have been developed for free products,[10] for right-angled Artin groups,[11] for the so-called deformation spaces of group actions[6] and in some other contexts.
• A base-pointed version of Outer space, called Auter space, for marked metric graphs with base-points, was constructed by Hatcher and Vogtmann in 1998.[12] The Auter space ${\displaystyle A_{n}}$  shares many properties in common with the Outer space, but ${\displaystyle A_{n}}$  only comes with an action of ${\displaystyle Aut(F_{n})}$ .

## References

1. ^ a b c Culler, Marc; Vogtmann, Karen (1986). "Moduli of graphs and automorphisms of free groups" (PDF). Inventiones Mathematicae. 84 (1): 91–119. doi:10.1007/BF01388734.
2. ^ Marshall M. Cohen, and Martin Lustig, Very small group actions on R-trees and Dehn twist automorphisms, Topology 34 (1995), 575–617
3. ^ Bestvina, Mladen; Feighn, Mark (1994). "Outer limits" (PDF).
4. ^ Guiradel, Vincent (2000). "Dynamics of ${\displaystyle Out(F_{n})}$  on the boundary of outer space". Annales Scientifiques de l'École Normale Supérieure. 33 (4): 433–465. doi:10.1016/S0012-9593(00)00117-8.
5. ^ a b Frédéric Paulin, The Gromov topology on R-trees. Topology and its Applications 32 (1989), no. 3, 197–221.
6. ^ a b Vincent Guirardel, Gilbert Levitt, Deformation spaces of trees. Groups, Geometry, and Dynamics 1 (2007), no. 2, 135–181.
7. ^ Francaviglia, Stefano; Martino, Armando (2011). "Metric properties of Outer Space". Publicacions Matemàtiques. arXiv:0803.0640v2.
8. ^ Francaviglia, Stefano; Martino, Armando (2012). "The isometry group of Outer Space". Advances in Mathematics. 231 (3–4): 1940–1973. arXiv:0912.0299. doi:10.1016/j.aim.2012.07.011.
9. ^ Mladen Bestvina, The topology of Out(Fn). Proceedings of the International Congress of Mathematicians, Vol. II (Beijing, 2002), pp. 373-384, Higher Ed. Press, Beijing, 2002; ISBN 7-04-008690-5.
10. ^ Vincent Guirardel, and Gilbert Levitt, The outer space of a free product. Proceedings of the London Mathematical Society 94 (2007), no. 3, 695–714
11. ^ Ruth Charney, Nathaniel Stambaugh, Karen Vogtmann, Outer space for untwisted automorphisms of right-angled Artin groups, arXiv:1212.4791, preprint, 2012
12. ^ Allen Hatcher, and Karen Vogtmann, Cerf theory for graphs. Journal of the London Mathematical Society 58 (1998), no. 3, 633–655.