Oka-Weil approximation theorem for polynomials with exponents in compact convex sets in $\mathbb R^n$
Let $S\subset\R^n_+$ be a compact convex set with $0\in S$, and let $\cP^S$ be the ring of polynomials whose exponents of the $m$-th degree lie in the dilates $mS$. Sigurðardóttir proved an Oka--Weil theorem for compact $\cP^S$-convex subsets of $(\C^*)^n$ and asked whether it holds for all compact $\cP^S$-convex subsets of $\C^n$. We answer this question affirmatively when the cone $\R_+S$ is rational polyhedral, for every $S$ when $n=2$, and for several classes of nonpolyhedral cones in all dimensions; in all these cases the compact sets may meet the coordinate hyperplanes. For arbitrary $S$ we prove that the Oka--Weil property of a compact $\cP^S$-convex set is equivalent to the compactness of its hull with respect to a finitely generated monomial subring of $\cP^S$, to a corona-type certificate, and to the existence of a bounded neighbourhood defined by finitely many inequalities $|p_j|<c_j$ with $p_j\in\cP^S$. The proofs use monomial charts, the classical Oka--Weil theorem in an auxiliary affine space, and elementary Gelfand theory. As applications we obtain an Oka--Stolzenberg criterion for $\cP^S$-hulls and the corresponding rational approximation theorem.