Spheres and Projections for $\mathrm{Out}(F_n)$

Abstract

The outer automorphism group Out(F_2g) of a free group on 2g generators naturally contains the mapping class group of a punctured surface as a subgroup. We define a subsurface projection of the sphere complex of the connected sum of n copies of S^1 x S^2 into the arc complex of the surface and use this to show that this subgroup is a Lipschitz retract of Out(F_2g). We also use subsurface projections to give a simple proof of a result of Handel and Mosher [HM10] stating that stabilizers of conjugacy classes of free splittings and corank 1 free factors in a free group Fn are Lipschitz retracts of Out(F_n).

32 pages, 9 figures

References in corpus (2)

• The free splitting complex of a free group I: Hyperbolicity
• Subfactor projections

Cited by in corpus (2)

• Hyperbolic graphs for free products, and the Gromov boundary of the graph of cyclic splittings
• The mapping class group of connect sums of $S^2\times S^1$