A combinatorial summation identity over the lattice of labelled hypertrees is established that allows one to gain concrete information on the Euler characteristics of various automorphism groups of free products of groups. In particular, we establish formulae for the Euler characteristics of: the group of Whitehead automorphisms Wh(*(n)(i=1)G(i)) when the G(i) are of finite homological type; Aut(*(n)(i=1)G(i)) and Out(*(n)(i=1)G(i)) when the G(i) are finite; and the palindromic automorphism groups of finite rank free groups.
Title
The Euler characteristic of the whitehead automorphism group of a free product
Jensen, C., J. McCammond and J. Meier. "The Euler characteristic of the whitehead automorphism group of a free product ." Transactions of the American Mathematical Society 359 (6): 2577-2595.