Superconvergence of a nonconforming low order finite element

This (incomplete nonconforming P2) element can be considered as an enriched parametric Q1rot element (Rannacher-Turek element). In difference to this Q1rot element, one obtains a supercloseness property and superconvergence in the H1 seminorm. Additionally, in the case of small diffusion parameters, the enrichment of the Q1rot element leads to a stabilization in streamline direction similar to SDFEM.