  A motion of point vortices with periodic boundary conditions is studied by
using Weierstrass zeta functions. Scattering and recoupling of a vortex pair by
a third vortex becomes remarkable when the vortex density is large. Clustering
of vortices with various initial conditions is quantitated by the $L$ function
used in point process theory in spatial ecology. It is shown that clustering
persists if it is initially clustered like an infinite row or a checkered
pattern.

Fluid Dynamics