Finiteness of Stationary Configurations of the Planar Four-vortex Problem. II

In an earlier paper \cite{yu2021Finiteness}, we showed that there are finitely many stationary configurations (consisting of equilibria, rigidly translating configurations, relative equilibria and collapse configurations) in the planar four-vortex problem. However, we only established finiteness of collapse configurations in the sense of prescribing a collapse constant. In this paper, by developing ideas of Albouy-Kaloshin and Hampton-Moeckel to do an analysis of the singularities, we further show that there really are finitely many collapse configurations in the four-vortex problem. This is an unexpectedly result, because the $N$-vortex problem has infinitely many collapse configurations for $N=3$ and for $N=5$. %to complete the work on finiteness of stationary configurations of the planar four-vortex problem. We also provide better upper bounds for collapse configurations than that in \cite{yu2021Finiteness}.