Free-by-cyclic group

In group theory, especially, in geometric group theory, the class of free-by-cyclic groups have been deeply studied as important examples. A group [math]\displaystyle{ G }[/math] is said to be free-by-cyclic if it has a free normal subgroup [math]\displaystyle{ F }[/math] such that the quotient group [math]\displaystyle{ G/F }[/math] is cyclic. In other words, [math]\displaystyle{ G }[/math] is free-by-cyclic if it can be expressed as a group extension of a free group by a cyclic group (NB there are two conventions for 'by'). Usually, we assume [math]\displaystyle{ F }[/math] is finitely generated and the quotient is an infinite cyclic group. Equivalently, we can define a free-by-cyclic group constructively: if [math]\displaystyle{ \varphi }[/math] is an automorphism of [math]\displaystyle{ F }[/math], the semidirect product [math]\displaystyle{ F \rtimes_\varphi \mathbb{Z} }[/math] is a free-by-cyclic group. An isomorphism class of a free-by-cyclic group is determined by an outer automorphism. If two automorphisms [math]\displaystyle{ \varphi, \psi }[/math] represent the same outer automorphism, that is, [math]\displaystyle{ \varphi = \psi\iota }[/math] for some inner automorphism [math]\displaystyle{ \iota }[/math], the free-by-cyclic groups [math]\displaystyle{ F \rtimes_\varphi \mathbb{Z} }[/math] and [math]\displaystyle{ F \rtimes_\psi \mathbb{Z} }[/math] are isomorphic.

Examples

The class of free-by-cyclic groups contains various groups as follow:

• A free-by-cyclic group is hyperbolic if and only if the attaching map is atoroidal.
• Some free-by-cyclic groups are hyperbolic relative to free-abelian subgroups. More generally, all free-by-cyclic groups are hyperbolic relative to a collection of subgroups that are free-by-cyclic for an automorphism of polynomial growth.
• Notably, there is a non-CAT(0) free-by-cyclic group.

References

• A. Martino and E. Ventura (2004), The Conjugacy Problem for Free-by-Cyclic Groups . Preprint from the Centre de Recerca Matemàtica, Barcelona, Catalonia, Spain.

• Feighn, Mark; Handel, Michael Mapping tori of free group automorphisms are coherent, Ann. Math., Volume 149 (1999) no. 3

• Ghosh, P. (2023). Relative hyperbolicity of free-by-cyclic extensions. Compositio Mathematica, 159(1), 153-183.

• F. Dahmani and R. Li, Relative hyperbolicity for automorphisms of free products and free groups, Journal of Topology and AnalysisVol. 14, No. 01, pp. 55-92 (2022)

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