Composite mixed finite elements on plane quadrilaterals

Mixed finite elements over a plane convex quadrilateral are obtained by assembling two Raviart-Thomas mixed finite elements over triangles. The macroelement is given by an eliminating procedure of the degrees of freedom related to the common edge to the two triangles. This procedure results in a finite element with a space of interpolating functions containing the polynomials of degree &lel, where l is the greater integer for which the same property is satisfied by the relevant Raviart-Thomas [Mathematical Aspects of Finite Element Methods, Roma 1975, I. Galligani and E. Magenes, Eds., Lecture Notes in Mathematics Vol. 606, Springer-Verlag, Berlin, 1975] mixed finite element. The interpolation error is estimated by means of the technique of almost equivalent affine element as given by Ciavaldini and Nedelec [Rev. Fr. Autom. Inf. Recher. Operationnelle Ser. Rouge R2, 29-45 (1974)].