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Dendrology of groups: An introduction. (English) Zbl 0649.20033
Essays in group theory, Publ., Math. Sci. Res. Inst. 8, 265-319 (1987).
[For the entire collection see Zbl 0626.00014.]
This expository paper is an informal but nonetheless valuable introduction to some aspects of group actions on trees. A large amount of material, due to many authors, is covered and only a brief outline can be given here.
The work is in six sections. Sections 1 and 2 are introductory: the first deals, from a topological point of view, with the Bass-Serre theory of group actions on ordinary (simplicial) trees, the second with its extension to generalized trees or $${\mathbb{R}}$$-trees arising from work of Lyndon, Chiswell and Tits. Section 3 formulates some questions on “which groups act and how”, and from then on the material becomes more specialized. Section 4 covers work of Morgan and the author on connections with hyperbolic geometry and ideas of Thurston. Sections 5 and 6 discuss partial solutions to the questions of section 3, the first concentrating on 3-manifold theory after Stallings, and the second on the rank-two case.
Reviewer: D.J.McCaughan

##### MSC:
 20F65 Geometric group theory 20-02 Research exposition (monographs, survey articles) pertaining to group theory 57M15 Relations of low-dimensional topology with graph theory 05C05 Trees 05C25 Graphs and abstract algebra (groups, rings, fields, etc.) 20E06 Free products of groups, free products with amalgamation, Higman-Neumann-Neumann extensions, and generalizations