  The motion of assemblies of point vortices in a periodic parallelogram can be
described by the complex position $z_j(t)$ whose time derivative is given by
the sum of the complex velocities induced by other vortices and the solid
rotation centered at $z_j$. A numerical simulation up to 100 vortices in a
square periodic box is performed with various initial conditions, including
single and double rows, uniform spacing, checkered pattern, and complete
spatial randomness. Point process theory in spatial ecology is applied in order
to quantify clustering of the distribution of vortices. In many cases,
clustering of the distribution persists after a long time if the initial
condition is clustered. In the case of positive and negative vortices with the
same absolute value of strength, the $L$ function becomes positive for both
types of vortices. Scattering or recoupling of pairs of vortices by a third
vortex is remarkable.
