In the
mathematical
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
subject of
geometric group theory
Geometric group theory is an area in mathematics devoted to the study of finitely generated groups via exploring the connections between algebraic properties of such group (mathematics), groups and topology, topological and geometry, geometric pro ...
, the Culler–Vogtmann Outer space or just Outer space of a
free group
In mathematics, the free group ''F'S'' over a given set ''S'' consists of all words that can be built from members of ''S'', considering two words to be different unless their equality follows from the group axioms (e.g. ''st'' = ''suu''−1' ...
''F''
''n'' is a
topological space
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called poin ...
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
Action may refer to:
* Action (narrative), a literary mode
* Action fiction, a type of genre fiction
* Action game, a genre of video game
Film
* Action film, a genre of film
* ''Action'' (1921 film), a film by John Ford
* ''Action'' (1980 fil ...
of the
group of outer automorphisms Out(''F''''n'') of ''F''
''n''. The Outer space was introduced in a 1986 paper
of
Marc Culler
Marc Edward Culler (born November 22, 1953) is an American mathematician who works in geometric group theory and low-dimensional topology. A native Californian, Culler did his undergraduate work at the University of California at Santa Barbara and ...
and
Karen Vogtmann
Karen Vogtmann (born July 13, 1949 in Pittsburg, California[''Biographies of Candidates 200 ...](_blank)
, and it serves as a free group analog of the
Teichmüller space
In mathematics, the Teichmüller space T(S) of a (real) topological (or differential) surface S, is a space that parametrizes complex structures on S up to the action of homeomorphisms that are isotopic to the identity homeomorphism. Teichmüll ...
of a hyperbolic surface. Outer space is used to study homology and cohomology groups of Out(''F''
''n'') and to obtain information about
algebra
Algebra () is one of the broad areas of mathematics. Roughly speaking, algebra is the study of mathematical symbols and the rules for manipulating these symbols in formulas; it is a unifying thread of almost all of mathematics.
Elementary ...
ic,
geometric
Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is ca ...
and dynamical properties of Out(''F''
''n''), of its
subgroup
In group theory, a branch of mathematics, given a group ''G'' under a binary operation ∗, a subset ''H'' of ''G'' is called a subgroup of ''G'' if ''H'' also forms a group under the operation ∗. More precisely, ''H'' is a subgroup ...
s and individual outer automorphisms of ''F''
''n''. The space ''X''
''n'' can also be thought of as the set of 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
was introduced in a 1986 paper
of
Marc Culler
Marc Edward Culler (born November 22, 1953) is an American mathematician who works in geometric group theory and low-dimensional topology. A native Californian, Culler did his undergraduate work at the University of California at Santa Barbara and ...
and
Karen Vogtmann
Karen Vogtmann (born July 13, 1949 in Pittsburg, California[''Biographies of Candidates 200 ...](_blank)
, inspired by analogy with the
Teichmüller space
In mathematics, the Teichmüller space T(S) of a (real) topological (or differential) surface S, is a space that parametrizes complex structures on S up to the action of homeomorphisms that are isotopic to the identity homeomorphism. Teichmüll ...
of a hyperbolic surface. They showed that the natural action of
on
is
properly discontinuous
In mathematics, a group action on a space is a group homomorphism of a given group into the group of transformations of the space. Similarly, a group action on a mathematical structure is a group homomorphism of a group into the automorphism g ...
, and that
is
contractible
In mathematics, a topological space ''X'' is contractible if the identity map on ''X'' is null-homotopic, i.e. if it is homotopic to some constant map. Intuitively, a contractible space is one that can be continuously shrunk to a point within th ...
.
In the same paper Culler and Vogtmann constructed an embedding, via the ''translation length functions'' discussed below, of
into the infinite-dimensional projective space
, where
is the set of nontrivial
conjugacy class
In mathematics, especially group theory, two elements a and b of a group are conjugate if there is an element g in the group such that b = gag^. This is an equivalence relation whose equivalence classes are called conjugacy classes. In other wo ...
es of elements of
. They also proved that the closure
of
in
is compact.
Later a combination of the results of Cohen and Lustig and of Bestvina and Feighn identified (see Section 1.3 of ) the space
with the space
of projective classes of "very small" minimal isometric actions of
on
-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
In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. The word i ...
between ''F''
''n'' and the
fundamental group
In the mathematical field of algebraic topology, the fundamental group of a topological space is the group of the equivalence classes under homotopy of the loops contained in the space. It records information about the basic shape, or holes, o ...
1(''R''
''n'', ''v'') of ''R''
''n''. From this point on we identify ''F''
''n'' and
1(''R''
''n'', ''v'') via this isomorphism.
A marking on ''F''
''n'' consists of a
homotopy equivalence
In topology, a branch of mathematics, two continuous functions from one topological space to another are called homotopic (from grc, ὁμός "same, similar" and "place") if one can be "continuously deformed" into the other, such a defo ...
''f'' : ''R''
''n'' → Γ where Γ is a finite
connected graph
In mathematics and computer science, connectivity is one of the basic concepts of graph theory: it asks for the minimum number of elements (nodes or edges) that need to be removed to separate the remaining nodes into two or more isolated subgrap ...
without degree-one and degree-two vertices. Up to a (free)
homotopy
In topology, a branch of mathematics, two continuous functions from one topological space to another are called homotopic (from grc, ὁμός "same, similar" and "place") if one can be "continuously deformed" into the other, such a deform ...
, ''f'' is uniquely determined by the isomorphism ''f''
# : , that is by an isomorphism
A metric graph is a finite connected graph
together with the assignment to every topological edge ''e'' of Γ of a positive
real number
In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every ...
''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 ''f''
1 : ''R''
''n'' → Γ
1 and ''f''
2 : ''R''
''n'' → Γ
2 are ''equivalent'' if there exists an isometry ''θ'' : Γ
1 → Γ
2 such that, up to free homotopy, we have ''θ'' o ''f''
1 = ''f''
2.
The Outer space ''X''
''n'' consists of
equivalence class
In mathematics, when the elements of some set S have a notion of equivalence (formalized as an equivalence relation), then one may naturally split the set S into equivalence classes. These equivalence classes are constructed so that elements a ...
es 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 ''e''
1, ..., ''e''
''k''. Let
:
be the standard (''k'' − 1)-dimensional open
simplex
In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given dimension. ...
in R
''k''.
Given ''f'', there is a natural map ''j'' : Δ
''k'' → ''X''
''n'', where for ''x'' = (''x''
1, ..., ''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
In mathematics, an injective function (also known as injection, or one-to-one function) is a function that maps distinct elements of its domain to distinct elements; that is, implies . (Equivalently, implies in the equivalent contrapositi ...
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 ''e''
1, ..., ''e''
''k''. Define Δ
''k''′ ⊆ R
''k'' as the set of all ''x'' = (''x''
1, ..., ''x''
''k'') ∈ R
''k'', such that
, such that each ''x''
''i'' ≥ 0 and such that the set of all edges ''e''
''i'' in ''
'' 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 ''
'' with ''x''
''i'' = 0 to obtain a new marking ''f''
1 : ''R''
''n'' → Γ
1 and then assigning to each surviving edge ''e''
''i'' of Γ
1 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 A covering of a topological space X is a continuous map \pi : E \rightarrow X with special properties.
Definition
Let X be a topological space. A covering of X is a continuous map
: \pi : E \rightarrow X
such that there exists a discrete spa ...
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
In mathematics, a metric space is a set together with a notion of '' distance'' between its elements, usually called points. The distance is measured by a function called a metric or distance function. Metric spaces are the most general setti ...
(''T'', ''d'') which is a
real tree In mathematics, real trees (also called \mathbb R-trees) are a class of metric spaces generalising simplicial trees. They arise naturally in many mathematical contexts, in particular geometric group theory and probability theory. They are also the s ...
. The fundamental group
1(Γ) acts on ''T'' by
covering transformations which are also isometries of (''T'', ''d''), with the quotient space ''T''/
1(Γ) = Γ. Since the
induced homomorphism In mathematics, especially in algebraic topology, an induced homomorphism is a homomorphism derived in a canonical way from another map. For example, a continuous map from a topological space ''X'' to a topological space ''Y'' induces a group homom ...
''f''
# is an isomorphism between ''F''
''n'' =
1(''R''
''n'') and
1(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 ''T''
1 and ''T''
2 are ''equivalent'' if there exists an ''F''
''n''-equivariant isometry between ''T''
1 and ''T''
2.
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:
:
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
. For this reason
is called the ''translation length'' of ''g''. For any ''g'', ''u'' in ''F''
''n'' we have
, that is the function
is constant on each
conjugacy class
In mathematics, especially group theory, two elements a and b of a group are conjugate if there is an element g in the group such that b = gag^. This is an equivalence relation whose equivalence classes are called conjugacy classes. In other wo ...
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'' =
1(''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
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
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
:
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
if and only if for every ''g'' in ''F''
''n'' we have
Gromov topology
Another topology on
is the so-called ''Gromov topology'' or the ''equivariant Gromov–Hausdorff convergence topology'', which provides a version of
Gromov–Hausdorff convergence
In mathematics, Gromov–Hausdorff convergence, named after Mikhail Gromov and Felix Hausdorff, is a notion for convergence of metric spaces which is a generalization of Hausdorff convergence.
Gromov–Hausdorff distance
The Gromov–Hausdorff ...
adapted to the setting of an isometric group action.
When defining the Gromov topology, one should think of points of
as actions of
on
-trees.
Informally, given a tree
, another tree
is "close" to
in the Gromov topology, if for some large finite subtrees of
and a large finite subset
there exists an "almost isometry" between
and
with respect to which the (partial) actions of
on
and
almost agree. For the formal definition of the Gromov topology see.
[Frédéric Paulin, ''The Gromov topology on R-trees''. '' Topology and its Applications'' 32 (1989), no. 3, 197–221.]
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.
[Vincent Guirardel, Gilbert Levitt, ''Deformation spaces of trees''. '' Groups, Geometry, and Dynamics'' 1 (2007), no. 2, 135–181.]
Action of Out(''F''''n'') on Outer space
The group
Out(''F''''n'') admits a natural right
action
Action may refer to:
* Action (narrative), a literary mode
* Action fiction, a type of genre fiction
* Action game, a genre of video game
Film
* Action film, a genre of film
* ''Action'' (1921 film), a film by John Ford
* ''Action'' (1980 fil ...
by
homeomorphism
In the mathematical field of topology, a homeomorphism, topological isomorphism, or bicontinuous function is a bijective and continuous function between topological spaces that has a continuous inverse function. Homeomorphisms are the isom ...
s on ''X''
''n''.
First we define the action of the
automorphism group
In mathematics, the automorphism group of an object ''X'' is the group consisting of automorphisms of ''X'' under composition of morphisms. For example, if ''X'' is a finite-dimensional vector space, then the automorphism group of ''X'' is the g ...
Aut(''F''
''n'') on ''X''
''n''. Let ''α'' ∈ Aut(''F''
''n'') be an
automorphism
In mathematics, an automorphism is an isomorphism from a mathematical object to itself. It is, in some sense, a symmetry of the object, and a way of mapping the object to itself while preserving all of its structure. The set of all automorphis ...
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 In mathematics, especially in algebraic topology, an induced homomorphism is a homomorphism derived in a canonical way from another map. For example, a continuous map from a topological space ''X'' to a topological space ''Y'' induces a group homom ...
at the
fundamental group
In the mathematical field of algebraic topology, the fundamental group of a topological space is the group of the equivalence classes under homotopy of the loops contained in the space. It records information about the basic shape, or holes, o ...
level is the automorphism ''α'' of ''F''
''n'' =
1(''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:
:
At the level of translation length functions the tree ''Tα'' is given as:
:
One then checks that for the above action of Aut(''F''
''n'') on Outer space ''X''
''n'' the subgroup of
inner automorphism
In abstract algebra an inner automorphism is an automorphism of a group, ring, or algebra given by the conjugation action of a fixed element, called the ''conjugating element''. They can be realized via simple operations from within the group itse ...
s 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 group
In the mathematical field of algebraic topology, the fundamental group of a topological space is the group of the equivalence classes under homotopy of the loops contained in the space. It records information about the basic shape, or holes, o ...
s isomorphic to ''F''
''n'' (that is, with the first
Betti number
In algebraic topology, the Betti numbers are used to distinguish topological spaces based on the connectivity of ''n''-dimensional simplicial complexes. For the most reasonable finite-dimensional spaces (such as compact manifolds, finite simplici ...
equal to ''n'') equipped with volume-one metric structures. The
quotient topology
In topology and related areas of mathematics, the quotient space of a topological space under a given equivalence relation is a new topological space constructed by endowing the quotient set of the original topological space with the quotient to ...
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
Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to:
* Interstate compact
* Blood compact, an ancient ritual of the Philippines
* Compact government, a type of colonial rule utilized in Britis ...
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
In mathematics, a topological space ''X'' is contractible if the identity map on ''X'' is null-homotopic, i.e. if it is homotopic to some constant map. Intuitively, a contractible space is one that can be continuously shrunk to a point within th ...
and the action of Out(''F''
''n'') on ''X''
''n'' is
properly discontinuous
In mathematics, a group action on a space is a group homomorphism of a given group into the group of transformations of the space. Similarly, a group action on a mathematical structure is a group homomorphism of a group into the automorphism g ...
, as was proved by Culler and
Vogtmann in their original 1986 paper
[ where Outer space was introduced.
* The space ''X''''n'' has ]topological dimension
In mathematics, the Lebesgue covering dimension or topological dimension of a topological space is one of several different ways of defining the dimension of the space in a
topologically invariant way.
Informal discussion
For ordinary Euclidean ...
3''n'' − 4. The reason is that if Γ is a finite connected graph without degree-one and degree-two vertices with fundamental group
In the mathematical field of algebraic topology, the fundamental group of a topological space is the group of the equivalence classes under homotopy of the loops contained in the space. It records information about the basic shape, or holes, o ...
isomorphic to ''F''''n'', then Γ has at most 3''n'' − 3 edges and it has exactly 3''n'' − 3 edges when Γ is trivalent. Hence the top-dimensional open simplex in ''X''''n'' has dimension 3''n'' − 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 2''n'' − 3, is Out(''F''''n'')-invariant and has compact quotient under the action of Out(''F''''n'').
Unprojectivized Outer space
The ''unprojectivized Outer space'' 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 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 has, including the coincidence of the three topologies (Gromov, axes, weak), and an -action. In addition, there is a natural action of on by scalar multiplication.
Topologically, is homeomorphic
In the mathematical field of topology, a homeomorphism, topological isomorphism, or bicontinuous function is a bijective and continuous function between topological spaces that has a continuous inverse function. Homeomorphisms are the isomor ...
to . In particular, is also contractible.
Projectivized Outer space
The projectivized Outer space is the quotient space under the action of on by scalar multiplication. The space is equipped with the quotient topology. For a tree its projective equivalence class is denoted . The action of on naturally quotients through to the action of on . Namely, for and put