Products of positive forms, linear matrix inequalities, and Hilbert 17th problem for ternary forms by Klerk, Etienne de., Pasechnik, Dmitrii V.

“…(N.S.), 37 (4) (2000) 407] (later proven by Artin [The Collected Papers of Emil Artin, Addison-Wesley Publishing Co., Inc., Reading, MA, London, 1965]) is that a form p is p.s.d. if and only if it can be decomposed into a sum of squares of rational functions. …”
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