A posteriori error estimates for non-conforming approximation of eigenvalue problems

We consider the approximation of eigenvalue problem for the Laplacian by the Crouzeix–Raviart non-conforming finite elements in two and three dimensions.Extending known techniques for source problems, we introduce a posteriori error estimators for eigenvectors and eigenvalues. We prove that the error estimator is equivalent to the energy norm of the eigenvector error up to higher order terms. Moreover, we prove that our estimator provides an upper bound for the error in the approximation of the first eigenvalue, also up to higher order terms.We present numerical examples of an adaptive procedure based on our error estimator in two and three dimensions. These examples show that the error in the adaptive procedure is optimal in terms of the number of degrees of freedom.