Abstract:
Given a class $\mathscr K$ of groups, we prove that the free product of a $\mathscr K$-group $A$ and a residually $\mathscr K$-group $B$ with amalgamated subgroup which is a retract of $B$ is a residually $\mathscr K$-group. We also obtain a sufficient condition for the root-class residuality of a generalized free product of two residually $\mathscr K$-groups with amalgamated subgroup which is a retract of one of the factors.