Ultraconvergence of ZZ patch recovery at mesh symmetry points (Q1412189)

The authors analyze mathematically the ultraconvergence property of the Zienkiewicz-Zhu (ZZ) gradient patch recovery techniques [cf. \textit{O. C. Zienkiewicz} and \textit{J. Z. Zhu}, Comput. Methods Appl. Mech. Eng. 101, No.~1--3, 207--224 (1992; Zbl 0779.73078)]. They use different smoothing strategies under the least-squares fitting for Q8 (eight-node serendipity element) and Q9 (nine-node tensor product element) finite elements. Some numerical tests are reported concerning the ultraconvergence property of the Q8 element. This is a nice and well written paper.