We prove that the quotient map from Aut(F_n) to Out(F_n) induces an
isomorphism on homology in dimension i for n at least 2i+4. This corrects an
earlier proof by the first author and significantly improves the stability
range. In the course of the proof, we also prove homology stability for a
sequence of groups which are natural analogs of mapping class groups of
surfaces with punctures. In particular, this leads to a slight improvement on
the known stability range for Aut(F_n), showing that its i-th homology is
independent of n for n at least 2i+2.Comment: Published by Algebraic and Geometric Topology at
http://www.maths.warwick.ac.uk/agt/AGTVol4/agt-4-54.abs.htm