Superconvergence phenomena have been observed numerically in the piecewise
Hermite bicubic orthogonal spline collocation solution of Poisson's equatio
n on a rectangle. The purpose of this article is to demonstrate theoretical
ly the superconvergent fourth-order accuracy in the first-order partial der
ivatives of the collocation solution at the partition nodes. (C) 1999 John
Wiley & Sons, Inc.