Equilibria of vortex type Hamiltonians on closed surfaces

Abstract: We prove the existence of critical points of vortex type Hamiltonians [ H(p_1,\ldots, p_N) = \sum_{{i,j=1},{i\ne j}}^N \Gamma_i\Gamma_jG(p_i,p_j)+\psi(p_1,\dots,p_N) ] on a closed Riemannian surface $(\Sigma,g)$ which is not homeomorphic to the sphere or the projective plane. Here $G$ denotes the Green function of the Laplace-Beltrami operator in $\Sigma$, $\psi:\Sigma^N\to\mathbb{R}$ may be any function of class $C^1$, and $\Gamma_1,\dots,\Gamma_N\in\mathbb{R}\setminus{0}$ are the vorticities. The Kirchhoff-Routh Hamiltonian from fluid dynamics corresponds to $\psi = -\sum_{i=1}^N \Gamma_i^2h(p_i,p_i)$ where $h:\Sigma\times\Sigma\to\mathbb{R}$ is the regular part of the Laplace-Beltrami operator. We obtain critical points $p=(p_1,\dots,p_N)$ for arbitrary $N$ and vorticities $(\Gamma_1,\dots,\Gamma_N)$ in $\mathbb{R}^N\setminus V$ where $V$ is an explicitly given algebraic variety of codimension 1.