Bass–Serre theory

Bass–Serre theory

Bass–Serre theory is a part of the mathematical subject of group theory that deals with analyzing the algebraic structure of groups acting by automorphisms on simplicial trees. The theory relates group actions on trees with decomposing groups as iterated applications of the operations of free product with amalgamation and HNN extension, via the notion of the fundamental group of a graph of groups.Bass–Serre theory can be regarded as one-dimentional version of the orbifold theory.


Bass–Serre theory was developed by Jean-Pierre Serre in 1970s and formalized in "Trees", Serre's seminal 1977 monograph (developed in collaboration with Hyman Bass) on the subject. [J.-P. Serre. "Arbres, amalgames, SL2". Rédigé avec la collaboration de Hyman Bass. Astérisque, No. 46. Société Mathématique de France, Paris, 1977] J.-P. Serre, "Trees". (Translated from the French by John Stillwell). Springer-Verlag, 1980. ISBN: 3-540-10103-9] . Serre's original motivation was to understand the structure of certain algebraic groups whose Bruhat–Tits buildings are trees. However, the theory quickly became a standard tool of geometric group theory and geometric topology, particularly the study of 3-manifolds. Subsequent work of Hyman BassH. Bass, "Covering theory for graphs of groups." Journal of Pure and Applied Algebra, vol. 89 (1993), no. 1–2, pp. 3–47] contributed substantially to the formalization and development of basic tools of the theory and currently the term "Bass–Serre theory" is widely used to describe the subject.

Mathematically, Bass–Serre theory builds on exploiting and generalizing the properties of two older group-theoretic constructions: free product with amalgamation and HNN extension. However, unlike the traditional algebraic study of these two constructions, Bass–Serre theory uses the geometric language of covering theory and fundamental groups. Graphs of groups, which are the basic objects of Bass–Serre theory, can be viewed as one-dimensional versions of orbifolds.

Apart from Serre's book, the basic treatment of Bass–Serre theory is available in the article of Bass, the article of Scott and Wall [Peter Scott and Terry Wall. "Topological methods in group theory." in: "Homological group theory (Proc. Sympos., Durham, 1977)", pp. 137–203, London Mathematical Society Lecture Notes Series, vol. 36, Cambridge University Press, Cambridge-New York, 1979; ISBN: 0-521-22729-1 ] and the books of Hatcher [A. Hatcher. "Algebraic topology." Cambridge University Press, Cambridge, 2002. ISBN: 0-521-79160-X; 0-521-79540-0 ] , Baumslag [G. Baumslag. "Topics in combinatorial group theory." Lectures in Mathematics ETH Zürich. Birkhäuser Verlag, Basel, 1993. ISBN:3-7643-2921-1] , Dicks and Dunwoody [W. Dicks, and M. J. Dunwoody. "Groups acting on graphs." Cambridge Studies in Advanced Mathematics, 17. Cambridge University Press, Cambridge, 1989. ISBN: 0-521-23033-0 ] and Cohen [Daniel E. Cohen. "Combinatorial group theory: a topological approach." London Mathematical Society Student Texts, 14. Cambridge University Press, Cambridge, 1989. ISBN: 0-521-34133-7] .

Basic set-up

Graphs in the sense of Serre

Serre's formalism of graphs is slightly different from the standard set up of Graph theory. Here a graph "A" consists of a "vertex set" "V", an "edge set" "E", an "edge reversal" map scriptstyle E o E, emapsto overline{e} such that scriptstyleoverline{e} e e and scriptstyleoverline{overline{e= e for every scriptstyle ein E, and an "initial vertex map" scriptstyle o: E o V. Thus in "A" every edge "e" comes equipped with its "formal inverse" scriptstyle overline{e}. The vertex "o"("e") is called the "origin" or the "initial vertex" of "e" and the vertex scriptstyle o(overline{e}) is called the "terminus" of "e" and is denoted "t"("e"). Both loop-edges (that is, edges "e" such that "o"("e")="t"("e")) and multiple edges are allowed. An "orientation" on "A" is a partition of "E" into the union of two disjoint subsets "E"+ and "E" so that for every edge "e" exactly one of the edges from the pair e, overline{e} belongs to "E"+ and the other belongs to "E".

Graphs of groups

A "graph of groups" A consists of the following data:
* A connected graph "A";
* An assignment of a "vertex group" "A""v" to every vertex "v" of "A".
* An assignment of an "edge group" "A""v" to every edge "e" of "A" so that we have scriptstyle A_e=A_{overline e} for every "e"∈"E".
* "Boundary monomorphisms" scriptstyle alpha_e: A_e o A_{o(e)} for all edges "e" of "A", so that each "αe" is an injective group homomorphism.

For every "e"∈"E" the map scriptstylealpha_{overline e}:A_e o A_{t(e)} is also denoted by "ωe".

Fundamental group of a graph of groups

There are two equivalent definitions of the notion of the fundamental group of a graph of groups: the first is a direct algebraic definition via an explicit group presentation (as a certain iterated application of amalgamated free products and HNN extensions), and the second using the language of groupoids.

The algebraic definition is easier to state:

First, choose a spanning tree "T" in "A" and an orientation scriptstyle E=E^+sqcup E^- on "A". The fundamental group of "A" with respect to "T", denoted scriptstylepi_1(mathbf A,T), is defined as the quotient of the free product :scriptstyle(ast_{vin V} A_v) ast F(E) where "F"("E") is a free group with free basis "E", subject to the following relations:
*scriptstyleoverline{e} alpha_e(g)e=omega_e(g) for every "e" in "E" and every scriptstyle gin A_e. (The so-called "Bass–Serre relation".)
*scriptstyle overline e e=1 for every scriptstyle ein E.
*scriptstyle e=1 for every edge "e" of the spanning tree "T".

There is also a notion of the fundamental group of "A" with respect to a base-vertex scriptstyle vin V, denoted scriptstylepi_1(mathbf A,v), which is defined using the formalism of groupoids. It turns out that for every choice of a base-vertex "v" and every spanning tree "T" in "A" the groups scriptstylepi_1(mathbf A,T) and scriptstylepi_1(mathbf A,v) are naturally isomorphic.

Fundamental groups of graphs of groups as iterations of amalgamated products and HNN-extensions

The group scriptstyle G=pi_1(mathbf A,T) defined above admits an algebraic description in terms of iterated amalgamated free products and HNN extensions. First, form a group "B" as a quotient of the free product

:scriptstyle(ast_{vin V} A_v)*F(E^+T)

subject to the relations
*"e"-1"α""e"("g")"e"="ω""e"("g") for every "e" in "E+T" and every scriptstyle gin A_e.
*"e"=1 for every "e" in "E"+"T".

This presentation can be rewritten as:scriptstyle B=ast_{vin V} A_v/{ m ncl}{alpha_e(g)=omega_e(g), ext{ where }ein E^+T, gin G_e}which shows that "B" is an iterated amalgamated free product of the vertex groups "A""v".

Then the group scriptstyle G=pi_1(mathbf A,T) has the presentation

:scriptstyle langle B, E^+(A-T)| e^{-1}alpha_e(g)e=omega_e(g) ext{ where }ein E^+(A-T), gin G_e angle ,

which shows that scriptstyle G=pi_1(mathbf A,T) is a multiple HNN extension of "B" with stable letters scriptstyle {e| ein E^+(A-T)}.


An isomorphism between a group "G" and the fundamental group of a graph of groups is called a "splitting" of "G". If the edge groups in the splitting come from a particular class of groups (e.g. finite, cyclic, abelian, etc), the splitting is said to be a "splitting over" that class.Thus a splitting where all edge groups are finite is called a splitting over finite groups.

Algebraically, a splitting of "G" with trivial edge groups corresponds to a free product decomposition :scriptstyle G=(ast A_v)ast F(X) where "F"("X") is a free group with free basis "X"="E"+("A-T") consisting of all positively oriented edges (with respect to some orientation on "A") in the complement of some spanning tree "T" of "A".

The normal forms theorem

Let "g" be an element of scriptstyle G=pi_1(mathbf A,T) represented as a product of the form:g=a_0e_1a_1dots e_na_n,where "e"1,..., "e""n" is a closed edge-path in "A" with the vertex sequence "v"0, "v"1,...,"vn"="v"0 (that is "v"0="o"("e"1), "v"n="t"("e""n") and "v""i"="t"("e""i")="o"("e""i"+1) for 0 < "i" < "n") and where a_iin A_{v_i} for "i" = 0, ..., "n".

Suppose that "g" = 1 in "G". Then
*either "n=0" and "a"0=1 in scriptstyle A_{v_0},
*or "n" > 0 and there is some 0 < "i" < "n" such that scriptstyle e_{i+1}=overline{e_i} and scriptstyle a_iin omega_{e_i}(A_{e_i}).

The normal forms theorem immediately implies that the canonical homomorphisms scriptstyle A_v o pi_1(mathbf A,T) are injective, so that we can think of the vertex groups "A""v" as subgroups of "G".

Bass–Serre covering trees

To every graph of groups A, with a specified choice of a base-vertex, one can associate a "Bass–Serre covering tree" scriptstyle ilde {mathbf A} , which is a tree that comes equipped with a natural group action of the fundamental group scriptstyle pi_1(mathbf A,v) without edge-inversions.Moreover, the quotient graph scriptstyle ilde {mathbf A}/pi_1(mathbf A,v) is isomorphic to "A".

Similarly, if "G" is a group acting on a tree "X" without edge-inversions (that is, so that for every edge "e" of "X" and every "g" in "G" we have ge e e), one can define the natural notion of a "quotient graph of groups" A. The underlying graph "A" of A is the quotient graph "X/G". The vertex groups of A are isomorphic to vertex stabilizers in "G" of vertices of "X" and the edge groups of A are isomorphic to edge stabilizers in "G" of edges of "X".

Moreover, if "X" was the Bass–Serre covering tree of a graph of groups A and if scriptstyle G=pi_1(mathbf A,v) then the quotient graph of groups for the action of "G" on "X" can be chosen to be naturally isomorphic to A.

Fundamental theorem of Bass–Serre theory

Let "G" be a group acting on a tree "X" without inversions. Let A be the quotient graph of groups and let "v" be a base-vertex in "A". Then "G" is isomorphic to the group pi_1(mathbf A,v) and there is an equivariant isomorphism between the tree "X" and the Bass–Serre covering tree ilde {mathbf A} and the tree "X". More precisely, there is a group isomorphism sigma: G o pi_1(mathbf A,v) and a graph isomorphism j:X o ilde {mathbf A} such that for every "g" in "G", for every vertex "x" of "X" and for every edge "e" of "X" we have "j"("gx") = "g" "j"("x") and "j"("ge") = "g" "j"("e").

One of the immediate consequences of the above result is the classic Kurosh subgroup theorem describing the algebraic structure of subgroups of free products.


Amalgamated free product

Consider a graph of groups A consisting of a single non-loop edge "e" (together with its formal inverse overline e) with two distinct end-vertices "u" = "o"("e") and "v" = "t"("e"), vertex groups "H"="A""u", "K"="A""v", an edge group "C"="A""e" and the boundary monomorphisms alpha=alpha_e:C o H, omega=omega_e:C o K.Then "T" = "A" is a spanning tree in "A" and the fundamental group pi_1(mathbf A,T) is isomorphic to the amalgamated free product

: G=Hast_C K=Hast K/{ m ncl}{alpha(c)=omega(c), cin C}.

In this case the Bass–Serre tree X= ilde{mathbf A} can be described as follows. The vertex set of "X" is the set of cosets

:VX= {gK:gin G}sqcup {gH:gin G}.

Two vertices "gK" and "fH" are adjacent in "X" whenever there exists kin K such that "fH" = "gkH" (or, equivalently, whenever there is hin H such that "gK" = "fhK").

The "G"-stabilizer of every vertex of "X" of type "gK" is equal to "gKg"-1 and the "G"-stabilizer of every vertex of "X" of type "gH" is equal to "gHg"-1. For an edge ["gH", "ghK"] of "X" its "G"-stabilizer is equal to "ghα"("C")"h"-1"g"-1.

For every cin C and hin H the edges ["gH", "ghK"] and ["gH, ghα"("c")"K"] are equal and the degree of the vertex "gH" in "X" is equal to the index ["H":"α"("C")] .Similarly, every vertex of type "gK" has degree ["K":"&omega;"("C")] in "X".

HNN extension

Let A be a graph of groups consisting of a single loop-edge "e" (together with its formal inverse overline e), a single vertex "v" = "o"("e") = "t"("e"), a vertex group "B"="A""v", an edge group "C"="A""e" and the boundary monomorphisms alpha=alpha_e:C o B, omega=omega_e:C o B.Then "T" = "v" is a spanning tree in "A" and the fundamental group pi_1(mathbf A,T) is isomorphic to the HNN extension

: G = langle B, e| e^{-1}alpha(c)e=omega(c), cin C angle.

with the base group "B", stable letter "e" and the associated subgroups "H"="α"("C"), "K"="&omega;"("C") in "B".The composition phi=omega circ alpha^{-1}:H o K is an isomorphism and the above HNN-extension presentation of "G" can be rewritten as

: G = langle B, e| e^{-1}he=phi(h), hin H angle .

In this case the Bass–Serre tree X= ilde{mathbf A} can be described as follows. The vertex set of "X" is the set of cosets VX= {gB:gin G}.

Two vertices "gB" and "fB" are adjacent in "X" whenever there exists bin B such that either "fB=gbeB" or "fB" = "gbe"−1"B". The "G"-stabilizer of every vertex of "X" is conjugate to "B" in "G" and the stabilizer of every edge of "X" is conjugate to "H" in "G". Every vertex of "X" has degree equal to ["B":"H"] + ["B":"K"] .

A graph with the trivial graph of groups structure

Let A be a graph of groups with underlying graph "A" such that all the vertex and edge groups in A are trivial. Let "v" be a base-vertex in "A". Then "&pi;"1(A,"v") is equal to the fundamental group "&pi;"1("A","v") of the underlying graph "A" in the standard sense of algebraic topology and the Bass-Serre covering tree ilde{mathbf A} is equal to the standard universal covering space ilde{A} of "A". Moreover, the action of "&pi;"1(A,"v") on ilde{mathbf A} is exactly the standard action of "&pi;"1("A","v") on ilde{A} by deck transformations.

Basic facts and properties

*If A is a graph of groups with a spanning tree "T" and if G=pi_1(mathbf{A},T), then for every vertex "v" of "A" the canonical homomorphism from "Av" to "G" is injective.
*If gin G is an element of finite order then "g" is conjugate in "G" to an element of finite order in some vertex group "Av".
*If Fle G is a finite subgroup then "F" is conjugate in "G" to a subgroup of some vertex group "Av".
*If the graph "A" is finite and all vertex groups "Av" are finite then the group "G" is "virtually free", that is, "G" contains a free subgroup of finite index.
*If "A" is finite and all the vertex groups "Av" are finitely generated then "G" is finitely generated.
*If "A" is finite and all the vertex groups "Av" are finitely presented and all the edge groups "Ae" are finitely generated then "G" is finitely presented.

Trivial and nontrivial actions

A graph of groups A is called "trivial" if "A" = "T" is already a tree and there is some vertex "v" of "A" such that A_v=pi_1(mathbf{A},A). This is equivalent to the condition that "A" is a tree and that for every edge "e" = ["u","z"] of "A" (with "o"("e") = "u", "t"("e") = "z") such that "u" is closer to "v" than "z" we have ["A""z":"&omega;""e"("A""e")] =1, that is "A""z"="&omega;""e"("A""e").

An action of a group "G" on a tree "X" without edge-inversions is called "trivial" if there exists a vertex "x" of "X" that is fixed by "G", that is such that "Gx" = "x". It is known that an action of "G" on "X" is trivial if and only if the quotient graph of groups for that action is trivial.

Typically, only nontrivial actions on trees are studied in Bass–Serre theory since trivial graphs of groups do not carry any interesting algebraic information, although trivial actions in the above sense (e. g. actions of groups by automorphisms on rooted trees) may also be interesting for other mathematical reasons.

One of the classic and still important results of the theory is a theorem of Stallings about ends of groups. The theorem states that a finitely generated group has more than one end if and only if this group admits a nontrivial splitting over finite subroups that is, if and only if the group admits a nontrivial action without inversions on a tree with finite edge stabilizers. [J. R. Stallings. "Groups of cohomological dimension one." in: "Applications of Categorical Algebra (Proc. Sympos. Pure Math., Vol. XVIII, New York, 1968)", pp. 124&ndash;128; American Mathematical Society, Providence, R.I, 1970. ]

An important general result of the theory states that if "G" is a group with Kazhdan's property (T) then "G" does not admit any nontrivial splitting, that is, that any action of "G" on a tree "X" without edge-inversions has a global fixed vertex. [Y. Watatani. "Property T of Kazhdan implies property FA of Serre." Mathematica Japonica, vol. 27 (1982), no. 1, pp. 97&ndash;103 ]

Hyperbolic length functions

Let "G" be a group acting on a tree "X" without edge-inversions.

For every "g"&isin;"G" put :ell_X(g)=min{ d(x,gx) | xin VX}.

Then ell_X(g), is called the "translation length" of "g" on "X".

The function:ell_X: G omathbb Z, quad gin Gmapsto ell_X(g)is called the "hyperbolic length function" or the "translation length function" for the action of "G" on "X".

Basic facts regarding hyperbolic length functions

*For "g"&isin;"G" exactly one of the following holds::(a) ell_X(g)=0, and "g" fixes a vertex of "G". In this caled "g" is called an "elliptic" element of "G".:(b) ell_X(g)>0, and there is a unique bi-infinite embedded line in "X", called the "axis" of "g" and denoted "Lg" which is "g"-invariant. In this case "g" acts on "Lg" by translation of magnitude ell_X(g), and the element "g"&isin;"G" is called "hyperbolic".
*If ell_X(G) e 0, then there exists a unique minimal "G"-invariant subtree "XG" of "X". Moreover "XG" is equal to the union of axes of hyperbolic elements of "G".

The length-function ell_X: G omathbb Z, is said to be "abelian" if it is a group homomorphism from "G" to mathbb Z, and "non-abelian" otherwise. Similarly, the action of "G" on "X" is said to be "abelian" if the associated hyperbolic length function is abelian and is said to be "non-abelian" otherwise.

In general, an action of "G" on a tree "X" without edge-inversions is said to be "minimal" if there are no proper "G"-invariant subtrees in "X".

An important fact in the theory says that minimal non-abelian tree actions are uniquely determined by their hyperbolic length functions:

Uniqueness Theorem

Let "G" be a group with two nonabelian minimal actions without edge-inversions on trees "X" and "Y". Suppose that the hyperbolic length functions ell_X, and ell_Y, on G are equal, that is ell_X(g)=ell_Y(g), for every "g"&isin;"G".Then the actions of "G" on "X" and "Y" are equal in the sense that there exists a graph isomorphism "f":"X"→"Y" which is "G"-equivariant, that is "f"("gx")="g" "f"("x") for every "g"&isin;"G" and every "x"&isin;"VX".

Important developments in Bass–Serre theory

Important developments in Bass–Serre theory in the last 30 years include:

*Various "accessibility results" for finitely presented groups that bound the complexity (that is, the number of edges) in a graph of groups decomposition of a finitely presented group, where some algebraic or geometric restrictions on the types of groups of groups considered are imposed. These results include:
**Dunwoody's theorem about "accessibility" of finitely presented groups [M. J. Dunwoody. "The accessibility of finitely presented groups." Inventiones Mathematicae vol. 81 (1985), no. 3, pp. 449&ndash;457] stating that for any finitely presented group "G" there exists a bound on the complexity of splittings of "G" over finite subgroups (the splittings are required to satisfy a technical assumption of being "reduced");
**Bestvina–Feighn "generalized accessibility" theorem stating that for any finitely presented group "G" there is a bound on the complexity of reduced splittings of "G" over "small" subgroups (the class of small groups includes, in particular, all groups that do not contain non-abelian free subgroups);
**"Acylindrical accessibility" results for finitely presented (Sela [Z. Sela. "Acylindrical accessibility for groups." Inventiones Mathematicae, vol. 129 (1997), no. 3, pp. 527−565] , Delzant [T. Delzant. "Sur l'accessibilité acylindrique des groupes de présentation finie." Université de Grenoble. Annales de l'Institut Fourier, vol. 49 (1999), no. 4, pp. 1215&ndash;1224] ) and finitely generated (Weidmann) groups which bound the complexity of the so-called "acylindrical" splittings, that is splittings where for their Bass–Serre covering trees the diameters of fixed subsets of nontrivial elements of G are uniformly bounded.
*The theory of "JSJ-decompositions" for finitely presented groups. This theory was motivated by the classic notion of JSJ decomposition in 3-manifold topology and was initiated, in the context of word-hyperbolic groups, by the work of Sela. JSJ decompositions are splittings of finitely presented groups over some classes of "small" subgroups (cyclic, abelian, noetherian, etc, depending on the version of the theory) that provide a canonical descriptions, in terms of some standard moves, of all splittings of the group over subgroups of the class. There are a number of versions of JSJ-decomposition theories:
**The initial version of Sela for cyclic splittings of torsion-free word-hyperbolic groups. [ Z. Sela, "Structure and rigidity in (Gromov) hyperbolic groups and discrete groups in rank $1$ Lie groups. II." Geometric and Functional Analysis, vol. 7 (1997), no. 3, pp. 561&ndash;593]
**Bowditch's version of JSJ theory for word-hyperbolic groups (with possible torsion) encoding their splittings over virtually cyclic subgroups. [B. H. Bowditch, "Cut points and canonical splittings of hyperbolic groups." Acta Mathematica, vol. 180 (1998), no. 2, pp. 145&ndash;186]
**The version of Rips and Sela of JSJ decompositions of torsion-free finitely presented groups encoding their splittings over free abelian subgroups. [E. Rips, and Z. Sela, "Cyclic splittings of finitely presented groups and the canonical JSJ decomposition." Annals of Mathematics (2) vol. 146 (1997), no. 1, pp. 53&ndash;109]
**The version of Dunwoody and Sageev of JSJ decompositions of finitely presented groups over noetherian subgroups. [M. J. Dunwoody, and M. E. Sageev, "JSJ-splittings for finitely presented groups over slender groups." Inventiones Mathematicae, vol. 135 (1999), no. 1, pp. 25&ndash;44. ]
**The version of Fujiwara and Papasoglu, also of JSJ decompositions of finitely presented groups over noetherian subgroups. [K. Fujiwara, and P. Papasoglu, "JSJ-decompositions of finitely presented groups and complexes of groups." Geometric and Functional Analysis, vol. 16 (2006), no. 1, pp. 70&ndash;125]
**A version of JSJ decomposition theory for finitely presented groups developed by Scott and Swarup. [Scott, Peter and Swarup, Gadde A."Regular neighbourhoods and canonical decompositions for groups." Astérisque No. 289 (2003). ]
*The theory of lattices in automorphism groups of trees. The theory of "tree lattices" was developed by Bass, Kulkarni and Lubotzky [ H. Bass, and R. Kulkarni. "Uniform tree lattices." Journal of the American Mathematical Society, vol. 3 (1990), no. 4, pp. 843&ndash;902 ] [A. Lubotzky. "Tree-lattices and lattices in Lie groups." in "Combinatorial and geometric group theory (Edinburgh, 1993)", pp. 217&ndash;232, London Mathematical Society Lecture Notes Series, vol. 204, Cambridge University Press, Cambridge, 1995; ISBN: 0-521-46595-8] by analogy with the theory of lattices in Lie groups (that is discrete subgroups of Lie groups of finite co-volume). For a discrete subgroup "G" of the automorphism group of a locally finite tree "X" one can define a natural notion of "volume" for the quotient graph of groups A as ::vol(mathbf A)=sum_{vin V} frac{1}. :The group "G" is called an "X-lattice" if vol(mathbf A). The theory of tree lattices turns out to be useful in the study of discrete subgroups of algebraic groups over non-archimedean local fields and in the study of Kac–Moody groups.
*Development of foldinds and Nielsen methods for approximating group actions on trees and analyzing their subgroup structure. [J.-R. Stallings. "Foldings of G-trees". in: "Arboreal Group Theory (Berkeley, CA, 1988)", Math. Sci. Res. Inst. Publ. 19 (Springer, New York, 1991), pp. 355&ndash;368. ISBN: 0-387-97518-7] M. Bestvina and M. Feighn. "Bounding the complexity of simplicial group actions on trees". Inventiones Mathematicae, vol. 103 (1991), no. 3, pp. 449&ndash;469 ] R. Weidmann. "The Nielsen method for groups acting on trees." Proceedings of the London Mathematical Society (3), vol. 85 (2002), no. 1, pp. 93&ndash;118] [I. Kapovich, R. Weidmann, and A. Miasnikov. "Foldings, graphs of groups and the membership problem." International Journal of Algebra and Computation, vol. 15 (2005), no. 1, pp. 95&ndash;128]
*The theory of ends and relative ends of groups, particularly various generalizations of Stallings theorem about groups with more than one end. [Scott, G. P. and Swarup, G. A. "An algebraic annulus theorem." Pacific Journal of Mathematics, vol. 196 (2000), no. 2, pp. 461&ndash;506] [M. J. Dunwoody, and E. L. Swenson, E. L. "The algebraic torus theorem."
Inventiones Mathematicae. vol. 140 (2000), no. 3, pp. 605&ndash;637
] [M. Sageev. "Codimension-1 subgroups and splittings of groups." Journal of Algebra, vol. 189 (1997), no. 2, pp. 377&ndash;389. ]
*Quasi-isometric rigidity results for groups acting on trees. [ P. Papasoglu. "Group splittings and asymptotic topology". Journal für die Reine und Angewandte Mathematik, vol. 602 (2007), pp. 1&ndash;16.]


There have been several generalizations of Bass-Serre theory:
*The theory of complexes of groups (see Haefliger [André Haefliger. "Complexes of groups and orbihedra." in: "Group theory from a geometrical viewpoint (Trieste, 1990)", pp. 504&ndash;540, World Sci. Publ., River Edge, NJ, 1991. ISBN: 981-02-0442-6 ] , Corson [Jon Corson. "Complexes of groups."
Proceedings of the London Mathematical Society (3) 65 (1992), no. 1, pp. 199&ndash;224.
] Bridson-Haefliger [ Martin R. Bridson, and André Haefliger. "Metric spaces of non-positive curvature." Grundlehren der Mathematischen Wissenschaften [Fundamental Principles of Mathematical Sciences] , 319. Springer-Verlag, Berlin, 1999. ISBN: 3-540-64324-9] ) provides a higher-dimensional generalization of Bass-Serre theory. The notion of a graph of groups is replaced by that of a complex of groups, where groups are assigned to each cell in a simplicial complex, together with monomorphisms between these groups corresponding to face inclusions (these monomorphisms are required to satisfy certain compatibility conditions). One can then define an analog of the fundamental group of a graph of groups for a complex of groups. However, in order for this notion to have good algebraic properties (such as embeddability of the vertex groups in it) and in order for a good analog for the notion of the Bass-Serre covering tree to exist in this context, one needs to require some sort of "non-positive curvature" condition for the complex of groups in question (see, for example [ Daniel T. Wise. The "residual finiteness of negatively curved polygons of finite groups." Inventiones Mathematicae, vol. 149 (2002), no. 3, pp. 579&ndash;617] [John R. Stallings. "Non-positively curved triangles of groups." in: "Group theory from a geometrical viewpoint (Trieste, 1990)", pp. 491&ndash;503, World Scientific Publishing, River Edge, NJ, 1991; ISBN: 981-02-0442-6] ).
*The theory of isometric group actions on real trees (or mathbb R-trees) which are metric spaces generalizing the graph-theoretic notion of a tree (graph theory). The theory was developed largely in the 1990s, where the work of Eliyahu Rips on the structure theory of "stable" group actions on mathbb R-trees played a key role (see Bestvina-Feighn [Mladen Bestvina, and Mark Feighn. "Stable actions of groups on real trees." Inventiones Mathematicae, vol. 121 (1995), no. 2, pp. 287&ndash;321] ). This structure theory assigns to a stable isometric action of a finitely generated group "G" a certain "normal form" approximation of that action by a stable action of "G" on a simplicial tree and hence a splitting of "G" in the sense of Bass-Serre theory. Group actions on real trees arise naturally in several contexts in geometric topology: for example as boundary points of the Teichmuller space [Richard Skora. "Splittings of surfaces." Bulletin of the American Mathematical Societ (N.S.), vol. 23 (1990), no. 1, pp. 85&ndash;90 ] (every point in the Thurston boundary of the Teichmuller space is represented by a measured geodesic lamination on the surface; this lamination lifts to the universal cover of the surface and a naturally dual object to that lift is an mathbb R-tree endowed with an isometric action of the fundamental group of the surface), as Gromov-Hausdorff limits of, appropriately rescaled, Kleinian group actions [Mladen Bestvina. "Degenerations of the hyperbolic space." Duke Mathematical Journal. vol. 56 (1988), no. 1, pp. 143&ndash;161 ] , and so on. The use of mathbb R-trees machinery provides substantial shortcuts in modern proofs of Thurston's Hyperbolization Theorem for Haken 3-manifolds.M. Kapovich. "Hyperbolic manifolds and discrete groups." Progress in Mathematics, 183. Birkhäuser. Boston, MA, 2001. ISBN: 0-8176-3904-7 ] [J.-P. Otal. "The hyperbolization theorem for fibered 3-manifolds."Translated from the 1996 French original by Leslie D. Kay. SMF/AMS Texts and Monographs, 7. American Mathematical Society, Providence, RI; Société Mathématique de France, Paris. ISBN: 0-8218-2153-9] Similarly, mathbb R-trees play a key role in the study of Culler-Vogtmann's Outer space [Marshall Cohen, and Martin Lustig. "Very small group actions on mathbb R-trees and Dehn twist automorphisms." Topology, vol. 34 (1995), no. 3, pp. 575&ndash;617] [Gilbert Levitt and Martin Lustig. "Irreducible automorphisms of Fn have north-south dynamics on compactified outer space." Journal de l'Institut de Mathématiques de Jussieu, vol. 2 (2003), no. 1, pp. 59&ndash;72 ] as well as in other areas of geometric group theory; for example, asymptotic cones of groups often have a tree-like structure and give rise to group actions on real trees. [Cornelia Drutu and Mark Sapir. "Tree-graded spaces and asymptotic cones of groups." (With an appendix by Denis Osin and Sapir.) Topology, vol. 44 (2005), no. 5, pp. 959&ndash;1058 ] [Cornelia Drutu, and Mark Sapir. "Groups acting on tree-graded spaces and splittings of relatively hyperbolic groups." Advances in Mathematics, vol. 217 (2008), no. 3, pp. 1313&ndash;1367 ] The use of mathbb R-trees, together with Bass-Serre theory, is a key tool in the work of Sela on solving the isomorphism problem for (torsion-free) word-hyperbolic groups, Sela's version of the JSJ-decomposition theory and the work of Sela on the Tarski Conjecture for free groups and the theory of limit groups. [Zlil Sela. "Diophantine geometry over groups and the elementary theory of free and hyperbolic groups." Proceedings of the International Congress of Mathematicians, Vol. II (Beijing, 2002), pp. 87&ndash;92, Higher Ed. Press, Beijing, 2002; ISBN: 7-04-008690-5] [Zlil Sela. "Diophantine geometry over groups. I. Makanin-Razborov diagrams." Publications Mathématiques. Institut de Hautes Études Scientifiques, No. 93 (2001), pp. 31&ndash;105]
*The theory of group actions on "Λ-trees", where "Λ" is an ordered abelian group (such as mathbb R or mathbb Z) provides a further generalization of both the Bass-Serre theory and the theory of group actions on mathbb R-trees (see Morgan [John W. Morgan. "Λ-trees and their applications." Bulletin of the American Mathematical Society (N.S.), vol. 26 (1992), no. 1, pp. 87&ndash;112.] , Alperin-BassR. Alperin and H. Bass. "Length functions of group actions on Λ-trees." in: Combinatorial group theory and topology (Alta, Utah, 1984), pp. 265&ndash;378, Annals of Mathematical Studies, 111, Princeton University Press, Princeton, NJ, 1987; ISBN: 0-691-08409-2] , Chiswell [Ian Chiswell. "Introduction to Λ-trees." World Scientific Publishing Co., Inc., River Edge, NJ, 2001. ISBN: 981-02-4386-3 ] ).


ee also

*Free product with amalgamation
*HNN extension
*Geometric group theory

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