Optimal Block Preconditioner for an Efficient Numerical Solution of the Elliptic Optimal Control Problems Using Gmres Solver

. Optimal control problems are a class of optimisation problems with partial diﬀer- ential equations as constraints. These problems arise in many application areas of science and engineering. The ﬁnite element method was used to transform the optimal control problems of an elliptic partial diﬀerential equation into a system of linear equations of saddle point form. The main focus of this paper is to characterise and exploit the structure of the coeﬃcient matrix of the saddle point system to build an eﬃcient numerical process. These systems are of large dimension, block, sparse, indeﬁnite and ill conditioned. The numerical solution of saddle point problems is a computational task since well known numerical schemes perform poorly if they are not properly preconditioned. The main task of this paper is to construct a preconditioner the mimic the structure of the system coeﬃcient matrix to accelerate the convergence of the generalised minimal residual method. Explicit expression of the eigenvalue and eigenvectors for the preconditioned matrix are derived. The main outcome is to achieve optimal convergence results in a small number of iterations with respect to the decreasing mesh size h and the changes in δ the regularisation problem parameters. The numerical results demonstrate the eﬀectiveness and performance of the proposed preconditioner compared to the other existing preconditioners and conﬁrm theoretical results.