Reduction and chaotic behavior of point vortices on a plane and a sphere
  We offer a new method of reduction for a system of point vortices on a plane and a sphere. This method is similar to the classical node elimination procedure. However, as applied to the vortex dynamics, it requires substantial modification. Reduction of four vortices on a sphere is given in more detail. We also use the Poincare surface-of-section technique to perform the reduction a four-vortex system on a sphere. 
A. V. Borisov, A. A. Kilin and I. S. Mamaev