Extremal positive semidefinite forms

Calderón, A.P.: A note on biquadratic forms. Linear Alg. Appl.7, 175–177 (1973) Choi, M.D.: Positive semidefinite biquadratic forms. Linear Alg. Appl.12, 95–100 (1975) Choi, M.D., Lam, T.Y.: Real zeros of positive semidefinite forms. In preparation Choi, M.D., Lam, T.Y.: Symmetric positive semidefinite forms and sums of squares. In preparation Hardy, G.H., Littlewood, J.E., Pólya, G.: Inequalities. Cambridge: Cambridge University Press 1934; second edition: 1952 Hilbert, D.: Über die Darstellung definiter Formen als Summe von Formenquadraten. Math. Ann.32, 342–350 (1888) (see also Ges. Abh., Bd. 2, 154–161. Berlin: Springer 1933) Koga, T.: Synthesis of finite passiven-ports with prescribed positive real matrices of several variables. IEEE Trans. Circ. Theory CT-15, 2–23 (1968) Motzkin, T.S.: The arithmetic-geometric inequality. In: Inequalities (Ed. O. Shisha) pp. 205–224. New York: Academic Press 1967 Robinson, R.M.: Some definite polynomials which are not sums of squares of real polynomials. In: Selected questions of Algebra and Logic. Acad. Sci. USSR, pp. 264–282, 1973 Rockafellar, R.T.: Convex Analysis. Princeton, NJ: Princeton University Press 1970