On the cyclic subgroup separability of free products of two groups with amalgamated subgroup

Abstract

Let $G$ be a free product of two groups with amalgamated subgroup, $\pi$ be either the set of all prime numbers or the one-element set \{$p$\} for some prime number $p$. Denote by $\Sigma$ the family of all cyclic subgroups of group $G$, which are separable in the class of all finite $\pi$-groups. Obviously, cyclic subgroups of the free factors, which aren't separable in these factors by the family of all normal subgroups of finite $\pi$-index of group $G$, the subgroups conjugated with them and all subgroups, which aren't $\pi^{\prime}$-isolated, don't belong to $\Sigma$. Some sufficient conditions are obtained for $\Sigma$ to coincide with the family of all other $\pi^{\prime}$-isolated cyclic subgroups of group $G$. It is proved, in particular, that the residual $p$-finiteness of a free product with cyclic amalgamation implies the $p$-separability of all $p^{\prime}$-isolated cyclic subgroups if the free factors are free or finitely generated residually $p$-finite nilpotent groups.