A functionφ:Nk0→Ris a moment (multi-)sequence if and only if the linearformLon the polynomial algebraR[x1,...,xk]defined byL(xn11...xnkk)=φ(n1,...,nk)is nonnegative on every polynomialpsuch that for some oddnatural numbermthe polynomialp(xm1,...,xmk)is a sum of squares of realpolynomials.
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