In this paper, a finite element space is presented on quadrilateral grids which can provide consistent discretization for the biharmonic equations. The space consists of piecewise quadratic polynomials and is of minimal degree for the variational problem.
introductionIn the study of qualitative and numerical analysis of partial differential equations and, in general, of approximation theory, we are often interested in the approximation of functions in Sobolev spaces by piecewise polynomials defined on a partition of the domain. In order for simpler interior structure, lower degree polynomials are often expected to be used. It is of theoretical and practical interests whether and how a minimal-degree finite element space can be constructed for certain problems, namely, particularly, whether and how a consistent finite element space can be constructed for H m elliptic problem with m-th degree polynomials.When the subdivision consists of simplexes, a systematic family minimal-degree family of nonconforming finite elements has been proposed by Wang and Xu [14] for 2m-th order elliptic partial differential equations in R n for any n m with polynomials with degree m. Known as Wang-Xu or Morley-Wang-Xu family, the elements have been playing bigger and bigger role in numerical analysis. The elements are constructed based on the perfect matching between the dimension of m-th degree polynomials and the dimension of (n −k)-subsimplexes with 1 k m. The generalization to the cases n < m is attracting more and more research interests, c.f., e.g., [16].When the subdivision consists of geometrical shapes other than simplex, the problem is more complicated. The minimal conforming element spaces have been constructed for H m problem on R n rectangle grids by Hu and Zhang [7], where Q k polynomials are used for 2k-th order problems. Some low-degree rectangle elements have been designed, such as the rectangular Morley element and incomplete P 3 element for biharmonic equation. It remains open if the degrees can be further reduced for consistent nonconforming element spaces with minimal degree. Generally, the cells of 2000 Mathematics Subject Classification. Primary 65L60, 65M60, 65N30, 31A30.