In the mathematical subject of geometric group theory, the Culler–Vogtmann Outer space or just Outer space of a free group F_n is a topological space consisting of the so-called "marked metric graph structures" of volume 1 on F_n. The Outer space, denoted X_n or CV_n, comes equipped with a natural action of the group of outer automorphisms Out(F_n) of F_n. 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(F_n) and to obtain information about algebraic, geometric and dynamical properties of Out(F_n), of its subgroups and individual outer automorphisms of F_n. The space X_n can also be thought of as the set of F_n-equivariant isometry types of minimal free discrete isometric actions of F_n on F_n on R-trees T such that the quotient metric graph T/F_n 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 F_n fix a "rose" R_n, that is a wedge, of n circles wedged at a vertex v, and fix an isomorphism between F_n and the fundamental group π₁(R_n, v) of R_n. From this point on we identify F_n and π₁(R_n, v) via this isomorphism.

A marking on F_n consists of a homotopy equivalence f : R_n → 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(R_n) → π₁(Γ), that is by an isomorphism F_n → π₁(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) 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 F_n consists of a marking f : R_n → Γ together with a metric graph structure L on Γ.

Two marked metric graph structures f₁ : R_n → Γ₁ and f₂ : R_n → 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 X_n consists of equivalence classes of all the volume-one marked metric graph structures on F_n.

Weak topology on the Outer space

Open simplices

Let f : R_n → 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, ..., e_k. 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 R^k.

Given f, there is a natural map j : Δ_k → X_n, where for x = (x₁, ..., x_k) ∈ Δ_k, the point j(x) of X_n is given by the marking f together with the metric graph structure L on Γ such that L(e_i) = x_i for i = 1, ..., k.

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

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

Closed simplices

Let f : R_n → 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, ..., e_k. Define Δ_k′ ⊆ R^k as the set of all x = (x₁, ..., x_k) ∈ R^k, such that ${\displaystyle \textstyle {\sum _{i=1}^{k}x_{i}=1))$, such that each x_i ≥ 0 and such that the set of all edges e_i in ${\displaystyle \Gamma }$ with x_i = 0 is a subforest in Γ.

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

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

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

Points of Outer space as actions on trees

Let x be a point in X_n given by a marking f : R_n → 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 F_n = π₁(R_n) and π₁(Γ), we also obtain an isometric action of F_n on T with T/F_n = 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 F_n-invariant subtrees.

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

Length functions

Give an action of F_n 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 F_n 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 X_n be represented by a marking f : R_n → Γ with a volume-one metric graph structure L on Γ. Let g ∈ F_n = π₁(R_n). 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 F_n define equal translation length functions on F_n then the two trees are F_n-equivariantly isometric. Hence the map ${\displaystyle T\mapsto \ell _{T))$ from X_n to the set of R-valued functions on F_n is injective.

One defines the length function topology or axes topology on X_n as follows. For every T in X_n, every finite subset K of F_n 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 X_n a basis of neighborhoods of T in X_n is given by the family V_T(K, ε) where K is a finite subset of F_n and where ε > 0.

Convergence of sequences in the length function topology can be characterized as follows. For T in X_n and a sequence T_i in X_n we have ${\displaystyle \lim _{i\to \infty }T_{i}=T}$ if and only if for every g in F_n 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 X_n coincide.^[6]

Action of Out(F_n) on Outer space

The group Out(F_n) admits a natural right action by homeomorphisms on X_n.

First we define the action of the automorphism group Aut(F_n) on X_n. Let α ∈ Aut(F_n) be an automorphism of F_n. Let x be a point of X_n given by a marking f : R_n → Γ with a volume-one metric graph structure L on Γ. Let τ : R_n → R_n be a homotopy equivalence whose induced homomorphism at the fundamental group level is the automorphism α of F_n = π₁(R_n). The element xα of X_n is given by the marking f ∘ τ : R_n → Γ with the metric structure L on Γ. That is, to get xα 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 X_n be a real tree with a minimal free and discrete co-volume-one isometric action of F_n. Let α ∈ Aut(F_n). As a metric space, Tα is equal to T. The action of F_n is twisted by α. Namely, for any t in T and g in F_n we have:

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

At the level of translation length functions the tree Tα 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(F_n) on Outer space X_n the subgroup of inner automorphisms Inn(F_n) is contained in the kernel of this action, that is every inner automorphism acts trivially on X_n. It follows that the action of Aut(F_n) on X_n quotients through to an action of Out(F_n) = Aut(F_n)/Inn(F_n) on X_n. namely, if φ ∈ Out(F_n) is an outer automorphism of F_n and if α in Aut(F_n) is an actual automorphism representing φ then for any x in X_n we have xφ = xα.

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

φx = xφ⁻¹.

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

Moduli space

The quotient space M_n = X_n/Out(F_n) 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 F_n (that is, with the first Betti number equal to n) equipped with volume-one metric structures. The quotient topology on M_n is the same as that given by the Gromov–Hausdorff distance between metric graphs representing points of M_n. The moduli space M_n is not compact and the "cusps" in M_n 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 X_n is contractible and the action of Out(F_n) on X_n is properly discontinuous, as was proved by Culler and Vogtmann in their original 1986 paper^[1] where Outer space was introduced.
• The space X_n 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 F_n, 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 X_n has dimension 3n − 4.
• Outer space X_n contains a specific deformation retract K_n of X_n, called the spine of Outer space. The spine K_n has dimension 2n − 3, is Out(F_n)-invariant and has compact quotient under the action of Out(F_n).

Unprojectivized Outer space

The unprojectivized Outer space ${\displaystyle cv_{n))$ consists of equivalence classes of all marked metric graph structures on F_n 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 F_n on R-trees, considered up to F_n-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 ${\displaystyle [T]=\{cT\mid c>0\}\subseteq cv_{n))$. 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 ${\displaystyle x,y}$ in X_n 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 ${\displaystyle x}$.

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

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

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 consist of (F_n-equivariant isometry classes of) all very small minimal isometric actions of F_n 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 ${\displaystyle \operatorname {Aut} (F_{n})}$.