  We consider an optimal control problem governed by an elliptic variational
inequality of the second kind. The problem is discretized by linear finite
elements for the state and a variational discrete approach for the control.
Based on a quadratic growth condition we derive nearly optimal a priori error
estimates. Moreover, we establish second order sufficient optimality conditions
that ensure a quadratic growth condition. These conditions are rather
restrictive, but allow us to construct a one-dimensional locally optimal
solution with reduced regularity, which serves as an exact solution for
numerical experiments.
