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The geometry of abstract groups and their splittings.

Charles Terence Clegg Wall — 2003

Revista Matemática Complutense

A survey of splitting theorems for abstract groups and their applications. Topics covered include preliminaries, early results, Bass-Serre theory, the structure of G-trees, Serre's applications to SL and length functions. Stallings' theorem, results about accessibility and bounds for splittability. Duality groups and pairs; results of Eckmann and collaborators on PD groups. Relative ends, the JSJ theorems and the splitting results of Kropholler and Roller on PD groups. Notions of quasi-isometry,...

Curves in P(C) with 1-dimensional symmetry.

A. A. du Plessis;  Charles Terence Clegg Wall — 1999

Revista Matemática Complutense

In a previous paper we showed that the existence of a 1-parameter symmetry group of a hypersurface X in projective space was equivalent to failure of versality of a certain unfolding. Here we study in detail (reduced) plane curves of degree d ≥ 3, excluding the trivial case of cones. We enumerate all possible group actions -these have to be either semisimple or unipotent- for any degree d. A 2-parameter group can only occur if d = 3. Explicit lists of singularities of the corresponding curves are...

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