  Many dynamical systems, such as the Lotka-Volterra predator-prey model and
the Euler equations for the free rotation of a rigid body, are PT symmetric.
The standard and well-known real solutions to such dynamical systems constitute
an infinitessimal subclass of the full set of complex solutions. This paper
examines a subset of the complex solutions that contains the real solutions,
namely, those having PT symmetry. The condition of PT symmetry selects out
complex solutions that are periodic.
