  Actuator location and design are important choices in controller design for
distributed parameter systems. Semi-linear partial differential equations model
a wide spectrum of physical systems with distributed parameters. It is shown
that under certain conditions on the nonlinearity and the cost function, an
optimal control input together with an optimal actuator choice exists.
First-order necessary optimality conditions are derived. The results are
applied to optimal actuator and controller design in a nonlinear railway track
model as well as semi-linear wave models.
