arXiv2026
Let $\mathcal{N}$ be a non-empty class of nilpotent groups closed under taking subgroups, quotient groups, finite direct products, and unrestricted direct powers of the form $\prod_{x \in X}Y_{x}$, where $X \in \mathcal{N}$ and $Y_{x}$ ($x \in X$) are isomorphic copies of a group $Y \in \mathcal{N}$. Let also the symbols $\mathcal{N}^{\mathfrak{tf}}$ and $\mathit{Poly}_{k}\mathcal{N}^{\mathfrak{tf}}$ denote the class of all torsion-free $\mathcal{N}$-groups and the class of polynilpotent groups such that $X \in \mathit{Poly}_{k}\mathcal{N}^{\mathfrak{tf}}$ if and only if $X$ has a subnormal series of length at most $k$ whose factors belong to $\mathcal{N}^{\mathfrak{tf}}$. We prove that if all free groups are residually $\mathcal{N}^{\mathfrak{tf}}$-groups and $2 \leqslant n < \infty$, then any tree product of $n$ residually $\mathcal{N}^{\mathfrak{tf}}$-groups with non-trivial torsion-free locally cyclic edge subgroups is residually a $(\mathit{Poly}_{n+1}\mathcal{N}^{\mathfrak{tf}})$-group. In particular, all cyclically-pinched one-relator groups are residually finitely generated $(\mathit{Poly}_{3}\overline{\mathcal{N}}\mbox{}^{\mathfrak{tf}})$-groups, where $\overline{\mathcal{N}}\mbox{}^{\mathfrak{tf}}$ is the class of all torsion-free nilpotent groups. We prove also that any tree product of $n$ $\mathcal{N}$-groups with non-trivial torsion-free locally cyclic edge subgroups has a homomorphism onto a $(\mathit{Poly}_{n-1}\mathcal{N}^{\mathfrak{tf}})$-group that is injective on all edge subgroups.