Stability of the finite elements 9/(4c + 1) and 9/5c for stationary Stokes equations

In this paper we study the stability of two quadrilateral elements 9/(4c + 1) and 9/5c for stationary Stokes equations. These two elements are very similar. However their stabilities are quite different. It is shown that the element 9/(4c + 1) is stable. The stability of the element 9/5c is shown mesh-dependent. Numerical tests are provided.