Hyperbolicity of Multiple Ascending HNN Extensions of Free Groups

Bestvina-Feighn-Handel show that for finitely many generic and independent hyperbolic automorphisms ${\phi }_1,\cdots ,{\phi }_r$ of $F_n$, the resulting extension $F_n⋊F_r$ is hyperbolic. This paper generalizes the above statement to the case where ${\phi }_1,\cdots ,{\phi }_r$ are hyperbolic non-surjective endomorphisms of $F_n$. In our case the output is a multiple HNN extension associated to a graph with one vertex and $r$ edges. All edge and vertex groups are isomorphic to $F_n$.