A complete classification of finite Morse index solutions to elliptic sine-Gordon equation in the plane

The elliptic sine-Gordon equation in the plane has a family of explicit multiple-end solutions (soliton-like solutions). We show that all the finite Morse index solutions belong to this family. We also prove they are non-degenerate in the sense that the corresponding linearized operators have no nontrivial bounded kernel. We then show that solutions with $2n$ ends have Morse index $n(n-1)\mathrm{/}2.$