2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/107740A 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.19 pages, 3 figures, to appear in Trans. Amer. Math. SocGroup TheoryCombinatorics20F28;20F65text