Some features of the nodal recovery of gradients from finite element approximations which produces superconvergence. (English). In: Kurzweil, Jaroslav (ed.): Equadiff 7. Proceedings of the 7th Czechoslovak Conference on Differential Equations and Their Applications held in Prague, 1989. BSB B.G. Teubner Verlagsgesellschaft, Leipzig, 1990. pp. 305-308