The Hanna Neumann Conjecture for graphs of free groups with cyclic edge groups

Abstract

The Hanna Neumann Conjecture (HNC) for a free group $G$ predicts that $\overlineχ(U\cap V)\leq \overlineχ (U)\overlineχ(V)$ for all finitely generated subgroups $U$ and $V$, where $\overlineχ(H) = \max\{-χ(H),0\}$ denotes the reduced Euler characteristic of $H$. A strengthened version of the HNC was proved independently by Friedman and Mineyev in 2011. Recently, Antolín and Jaikin-Zapirain introduced the $L^2$-Hall property and showed that if $G$ is a hyperbolic limit group that satisfies this property, then $G$ satisfies the HNC. Antolín and Jaikin-Zapirain established the $L^2$-Hall property for free and surface groups, which Brown and Kharlampovich extended to all limit groups. In this article, we prove the $L^2$-Hall property for graphs of free groups with cyclic edge groups that are hyperbolic relative to virtually abelian subgroups. We also give another proof of the $L^2$-Hall property for limit groups. As a corollary, we show that all these groups satisfy a strengthened version of the HNC.