Infinite dimensional extension of A.P. Calderon's theorem on positive semidefinite biquadratic forms
(Lobachevskii Journal of Mathematics, Volume X)
We extend to the infinite dimensional separable real Hilbert spaces a theorem of A. P. Calderòn which says that, if m=2 or n=2, then every positive semidefinite biquadratic form on Rm× Rn is a sum of squares of bilinear forms.