Borel measures with a density on a compact semi-algebraic set

Abstract

Let $K\subset R^n$ be a compact basic semi-algebraic set. We provide a necessary and sufficient condition (with no a priori bounding parameter) for a real sequence $y=(y_\alpha)$, $\alpha\in N^n$, to have a finite representing Borel measure absolutely continuous w.r.t. the Lebesgue measure on $K$, and with a density in $\cap_{p=1}^\infty L_p(K)$. With an additional condition involving a bounding parameter, the condition is necessary and sufficient for existence of a density in $L_\infty(K)$. Moreover, nonexistence of such a density can be detected by solving finitely many of a hierarchy of semidefinite programs. In particular, if the semidefinite program at step $d$ of the hierarchy has no solution then the sequence cannot have a representing measure on $K$ with a density in $L_p(K)$ for any $p\geq 2d$.