Stationary and translating relative equilibria of point vortices in the plane are studied. It is shown that stationary equilibria of a system containing point vortices with arbitrary choice of circulations can be described with the help of the Tkachenko equation. It is obtained that the Adler -Moser polynomial are not unique polynomial solutions of the Tkachenko equation. A generalization of the Tkachenko equation to the case of translating relative equilibria is derived. It is shown that the generalization of the Tkachenko equation possesses polynomial solutions with degrees that are not triangular numbers.
IntroductionThe problem of finding stationary and relative equilibrium solutions to Helmholtz's equations describing motion of point vortices in the plane has been attracting much attention during recent years [1][2][3][4][5][6][7][8][9]. The so-called "polynomial method" is often used while studying this problem [1]. According to this method one introduces polynomials with roots at vortex positions. For example, it was found by Stieltjes that the generating polynomial of N identical point vortices on a line is essentially the Nth Hermite polynomial. Considering equilibrium of point vortices with equal in absolute value circulations leads to an ordinary differential equation first discovered by Tkachenko [1]. It is well known that the Adler -Moser polynomials give polynomial solutions to the Tkachenko equation. Originally these polynomials arose in 1 the theory of one of the most famous soliton equation, the Korteweg -de Vries equation [10]. This fact provides a remarkable and rather unexpected connection between the dynamics of point vortices and the theory of integrable partial differential equations [11]. Not long ago analogous connections between equilibria of point vortices with circulations Γ, −2Γ and rational solutions of the Sawada -Kotera and the the Kaup -Kupershmidt equations was established [12].In this article we study stationary and translating relative equilibria of multivortex systems with circulations Γ 1 , . . ., Γ N . Our aim is to show that the stationary case can be described with the help of the Tkachenko equation and the translating case is reducible to a generalization of the Tkachenko equation. As a consequence of our results we obtain that the Adler -Moser polynomials are not unique polynomial solutions of the Tkachenko equation.This article is organized as follows. In section 2 we consider stationary equilibria of point vortices with arbitrary choice of circulations. We derive an ordinary differential equation satisfied by generating polynomials of arrangements and transform this equation to the Tkachenko equation. Section 3 is devoted to the translating case. We find an ordinary differential equation satisfied by generating polynomials of the vortices and reduce the resulting equation to the generalization of the Tkachenko equation. In section 4 we study properties of the Tkachenko equation and the generalized Tkachenko equation. Quite unexpectedly it is possible to constr...