Morley finite element method for the eigenvalues of the biharmonic operator

This paper studies the nonconforming Morley finite element approximation of the eigenvalues of the biharmonic operator. A new C1 conforming companion operator leads to an L2 error estimate for the Morley finite element method (FEM) which directly compares the L2 error with the error in the energy norm and, hence, can dispense with any additional regularity assumptions. Furthermore, the paper presents new eigenvalue error estimates for nonconforming finite elements that bound the error of (possibly multiple or clustered) eigenvalues by the approximation error of the computed invariant subspace. An application is the proof of optimal convergence rates for the adaptive Morley FEM for eigenvalue clusters.