Sums of 2m-th powers of rational functions in one variable over real closed fields. T.Y. Lam; A. Prestel; M.D. Choi — 1996 Mathematische Zeitschrift
The Pythagoras number of some affine algebras and local algebras. T.Y. Lam; M.D. Choi; Z.D. Dai — 1982 Journal für die reine und angewandte Mathematik