1. On the representation of rational functions as sums of squares;Cassels;Acta Arith.,1964
2. Choi, M.D., and Lam, T.-Y., An old question of Hilbert, Proc. Conf. on Quadratic Forms–1976 (Queen's Papers on Pure and Applied Math., No. 16 (G. Orzech, ed.), Queen's Univ., Kingston, Ontario, 1977), 385–405
3. Extremal positive semidefinite forms;Choi;Math. Ann.,1977
4. Delzell, C.N., A constructive, continuous solution to Hilbert's 17th problem, and other results in semi-algebraic geometry, Ph.D. dissertation, Stanford Univ., 1980 (Univ. Microfilms International, Order No. 8024640); cf. also Dissertation Abstracts International 41, no. 5, 1980
5. A continuous, constructive solution to Hilbert's 17th problem, in preparation (a). (See also a preliminary abstract in AMS Abstracts 2(1) (Jan. 1981), #783–12-28.)