On the finite index subgroups of Houghton's groups

An erratum has been added to resolve an issue raised by Professor Derek Holt. This appears after the original paper, and also includes two new results. Original abstract: Houghton’s groups $H_2,H_3,\dots$ are certain infinite permutation groups acting on a countably infinite set; they have been studied, among other things, for their finiteness properties. In this note we describe all of the finite index subgroups of each Houghton group, and their isomorphism types. Using the standard notation that $d(G)$ denotes the minimal size of a generating set for $G$ we then show, for each $n\in \{2,3,\dots \}$ and $U$ of finite index in $H_n$, that $d(U)\in \{d(H_n),d(H_n)+1\}$ and characterise when each of these cases occurs.