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The accessibility of finitely presented groups. (English) Zbl 0572.20025
A finitely generated group G has more than one end, $$e(G>1$$, if there exists a free action of G, with finite quotient, on a simplicial complex K with more than one end. Stallings proved that $$e(G)>1$$ iff G splits over a finite subgroup C as a nontrivial free product with amalgamation resp. HNN-extension, $$G=A*_ CB$$ or $$G=A*_{\{C,t\}}$$ (or equivalently, G acts on a tree such that G fixes no vertex and all edge-stabilizers are finite). Now if e(A) or $$e(B)>1$$ this splitting process can be iterated. A f.g. group G is called accessible if this splitting process stops after finitely many steps, or equivalently, G is the fundamental group of a (finite) graph of groups in which every edge group is finite and every vertex group has at most one end. For example, by Grushko’s theorem every f.g. torsion-free group is accessible. The conjecture is that every f.g. group G is accessible.
In the present paper it is shown that finitely presented groups are accessible. The proof is geometric-combinatorial, by splitting an action of G on a suitable simplicial complex K with finite quotient along certain 1-dim. finite subcomplexes which generalize simple closed curves on surfaces. The main point is that this splitting process terminates after finitely many steps; this is analogous to an argument of Kneser who showed that in a compact 3-manifold there are only finitely many disjoint non-parallel embedded 2-spheres. As noted in the introduction, similar methods can be applied to prove the equivariant loop and sphere theorems of Meeks-Yau, thus replacing the minimal surface techniques they used by combinatorial ones [see the author’s paper in Bull. Lond. Math. Soc. 17, 437-448 (1985)].
Reviewer: B.Zimmermann

##### MSC:
 20F65 Geometric group theory 57M20 Two-dimensional complexes (manifolds) (MSC2010) 05C25 Graphs and abstract algebra (groups, rings, fields, etc.) 57M05 Fundamental group, presentations, free differential calculus 20F05 Generators, relations, and presentations of groups 20E06 Free products of groups, free products with amalgamation, Higman-Neumann-Neumann extensions, and generalizations
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