On root-class residuality of generalized free products
  Root-class residuality of free product of root-class residual groups is demonstrated. A sufficient condition for root-class residuality of generalized free product $G$ of groups $A$ and $B$ amalgamating subgroups $H$ and $K$ through the isomorphism $\phi$ is derived. For the particular case when $A=B$, $H=K$ and $\phi$ is the identity mapping, it is shown that group $G$ is root-class residual if and only if $A$ is root-class residual and subgroup $H$ of $A$ is root-class closed. These results are extended to generalized free product of infinitely many groups amalgamating a common subgroup. 
D. N. Azarov and D. Tieudjo