Analytic Versus Algebraic Density of Polynomials

Abstract

We show that under very mild conditions on a measure $\mu$ on the interval $[0,\infty)$, the span of $\{x^k\}_{k=n}^{\infty}$ is dense in $L^2(\mu)$ for any $n=0,1,\ldots$. We present two different proofs of this result, one based on the density index of Berg and Thill and one based on the Hilbert space $L^2(\mu)\oplus \mathbb{C}^{n+1}$. Using the index of determinacy of Berg and Dur\'an we prove that if the measure $\mu$ on $\mathbb{R}$ has infinite index of determinacy then the polynomial ideal $R(x)\mathbb{C}[x]$ is dense in $L^2(\mu)$ for any polynomial $R$ with zeros having no mass under $\mu$.