Postprocessing and higher order convergence of the mixed finite element approximations of biharmonic eigenvalue problems

A new procedure for accelerating the convergence of mixed finite element approximations of the eigenpairs and of the biharmonic operator is proposed. It is based on a postprocessing technique that involves an additional solution of a source problem on an augmented finite element space. This space could be obtained either by substantially refining the grid, the two-grid method, or by using the same grid but increasing the order of polynomials by one, the two-space method. The numerical results presented and discussed in the paper illustrate the efficiency of the postprocessing method.