Errata and Addenda to: "Hydrodynamic Vortex on Surfaces" and "The motion of a vortex on a closed surface of constant negative curvature"

Abstract

The two papers in the title contain some equations that are not complete. The missing terms, which are of topological origin, were recently unveiled by Bjorn Gustafsson. In this note we present the equations of Gustafsson in the case of a single vortex in a compact boundaryless surface, and show that many conclusions we have reached with the incomplete equations remain valid. It seems that the the extra-terms in Gustafsson's equations can be explicitly written in terms of elementary functions only for the two-torus, and this is done at the end of this note.