For the optimal control problems bound by the time-periodic eddy current equation, we build a class of complex rotating block triangular preconditioners based on the BAS preconditioning matrix. The corresponding preconditioned matrices' spectrum characteristics are examined. The application of these complicated rotated block triangular preconditioners to quicken Krylov subspace iteration techniques demonstrates their potential to be competitive with and even more effective than the BAS preconditioner.
Ni et al. (Thu,) studied this question.