We study the cocircular relative equilibria (planar central configurations) in the four-vortex
problem using methods suggested by the study of cocircular central configurations in the
Newtonian four-body problem in recent work of Cors and Roberts. Using mutual distance
coordinates, we show that the set of four-vortex relative equilibria is a two-dimensional surface
with boundary curves representing kite configurations, isosceles trapezoids, and degenerate
configurations with one zero vorticity. We also show that there is a constraint on the signs of
the vorticities in these configurations; either three or four of the vorticities must have the same
sign, in contrast to the noncocircular cases studied by Hampton, Roberts, and Santoprete.
Keywords
relative equilibria, vortices, central configurations