Geodesic
Infinite dimensional extension of A. P. Calderón's theorem on positive semidefinite biquadraric forms
Lobachevskii journal of mathematics, Tome 10 (2002), pp. 3-8
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We extend to the infinite dimensional separable real Hilbert spaces a theorem of A. P. Calderón which says that, if $m=2$ or $n=2$, then every positive semidefinite biquadratic form on $\mathbf R^m\times\mathbf R^n$ is a sum of squares of bilinear forms.
Keywords: biquadratic form, positive operator, Hilbert–Schmidt operator. 
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B. Aqzzouz; M. Kadiri. Infinite dimensional extension of A. P. Calderón's theorem on positive semidefinite biquadraric forms. Lobachevskii journal of mathematics, Tome 10 (2002), pp. 3-8. http://geodesic.mathdoc.fr/item/LJM_2002_10_a0/

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