A preconditioner for optimal control problems, constrained by Stokes equation with a time-harmonic control

In this article we construct an efficient preconditioner for solving the algebraic systems arising from discretized optimal control problems with time-periodic Stokes equations, based on a preconditioning technique for stationary Stokes-constrained optimal control problems, considered in an earlier paper by the authors. A simplified analysis of the derivation of the preconditioner and its properties is presented. The preconditioner is fully parameter-independent and the condition number of the corresponding preconditioned matrix is bounded by 2. The so-constructed preconditioner is favourably compared with another robust preconditioner for the same problem.