More on the Density of Analytic Polynomials in Abstract Hardy Spaces

Abstract

Let $\{F_n\}$ be the sequence of the Fej\'er kernels on the unit circle $\mathbb{T}$. The first author recently proved that if $X$ is a separable Banach function space on $\mathbb{T}$ such that the Hardy-Littlewood maximal operator $M$ is bounded on its associate space $X'$, then $\|f*F_n-f\|_X\to 0$ for every $f\in X$ as $n\to\infty$. This implies that the set of analytic polynomials $\mathcal{P}_A$ is dense in the abstract Hardy space $H[X]$ built upon a separable Banach function space $X$ such that $M$ is bounded on $X'$. In this note we show that there exists a separable weighted $L^1$ space $X$ such that the sequence $f*F_n$ does not always converge to $f\in X$ in the norm of $X$. On the other hand, we prove that the set $\mathcal{P}_A$ is dense in $H[X]$ under the assumption that $X$ is merely separable.