Abstract: We consider the first boundary value problem of the second order elliptic equation and serendipity rectangular elements. Papers [2,3,9] proved that the gradients of finite element solution possess superconvergence at Gaussianpoint. In this paper, we extend the results in papers [2,3,9] in the sense that the coefficients of the elliptic equations are discontinuous on a curve S which lies in the bounded domain Ω.

Key words: impacting-rub, bending-torsion coupling, chaos, period adding