1. Artin, E.: Über die Zerlegung definiter Funktionen in Quadra, Quadrate. Abhandlungen aus dem Mathematischen Seminar der Universität Hamburg
2. Bochnak, J., Coste, M., Roy, M.-F.: Real Algebraic Geometry. Springer, Berlin (1998)
3. Chevillard, S., Harrison, J., Joldes, M., Lauter, Ch.: Efficient and accurate computation of upper bounds of approximation errors. Theor. Comput. Sci. 412, 1523–1543 (2011)
4. Choi, M.-D., Lam, T.Y.: An old question of Hilbert. In: Orzech, G. (ed.) Proceedings of Quadratic Forms Conference, Queen’s University, vol. 46, pp. 385–405. Queen’s Papers in Pure and Applied Mathematics (1977)
5. Choi, M.-D., Lam, T.Y.: Extreme positive semidefinite forms. Math. Ann. 231, 1–18 (1977)