Mortar method for nonconforming finite elements

A mortar method for second-order elliptic problems is considered using the nonconforming finite element proposed by Douglas et al. [J. Douglas, J.E. Santos, D. Sheen, X. Ye, Nonconforming Galerkin methods based on quadrilateral elements for second order elliptic problems, M2AN Math. Model. Numer. Anal. 33 (1999) 747-770]. The error estimate of optimal order is proved in the broken energy norm. Numerical experiments which confirm the convergence analysis are provided.