Holomorphic approximation by polynomials with exponents restricted to a convex cone

Abstract

We study approximations of holomorphic functions of several complex variables by proper subrings of the polynomials. The subrings in question consist of polynomials of several complex variables whose exponents are restricted to a prescribed convex cone $\mathbb{R}_+S$ for some compact convex $S\in \mathbb{R}^n_+$. Analogous to the polynomial hull of a set, we denote the hull of $K$ with respect to the given ring by can define hulls of a set $K$ with respect to the given ring, here denoted $\widehat K{}^S$. By studying an extremal function $V^S_K(z)$, we show a version of the Runge-Oka-Weil Theorem on approximation by these subrings on compact subsets of $\mathbb{C}^{*n}$ that satisfy $K= \widehat K{}^S$ and $V^{S*}_K|_K=0$. We show a sharper result for compact Reinhardt sets $K$, that a holomorphic function is uniformly approximable on $\widehat K{}^S$ by members of the ring if and only if it is bounded on $\widehat K{}^S$. We also show that if $K$ is a compact Reinhardt subsets of $\mathbb{C}^{*n}$, then we have $V^S_K(z)=\sup_{s\in S} (\langle s ,{\operatorname{Log}\, z}\rangle- \varphi_A(s)) $, where $\varphi_A$ is the supporting function of $A=\operatorname{Log}\, K= \{(\log|z_1|,\dots, \log|z_n|) \,;\, z\in K\}$.