Rational Approximation, Hardy Space - Decomposition of Functions in $L_p,p<1$: Further Results in Relation to Fourier Spectrum Characterization of Hardy Spaces

Abstract

Subsequent to our recent work on Fourier spectrum characterization of Hardy spaces $H^p(\mathbb{R})$ for the index range $1\le p\le \infty ,$ in this paper we prove further results on rational Approximation, integral representation and Fourier spectrum characterization of functions in the Hardy spaces $H^p(\mathbb{R}),0<p\le \infty ,$ with particular interest in the index range $0<p\le 1.$ We show that the set of rational functions in $H^p({\mathbb{C}}_{+1})$ with the single pole $-i$ is dense in $H^p({\mathbb{C}}_{+1})$ for $0<p<\infty .$ Secondly, for $0<p<1$, through rational function approximation we show that any function $f$ in $L^p(\mathbb{R})$ can be decomposed into a sum $g+h$, where $g$ and $h$ are, in the $L^p(\mathbb{R})$ convergence sense, the non-tangential boundary limits of functions in, respectively, $H^p({\mathbb{C}}_{+1})$ and $H^p({\mathbb{C}}_{-1}),$ where $H^p({\mathbb{C}}_k)(k=\pm 1)$ are the Hardy spaces in the half plane ${\mathbb{C}}_k=\{z=x+iy:ky>0\}$. We give Laplace integral representation formulas for functions in the Hardy spaces $H^p,$ $0<p\le 2.$ Besides one in the integral representation formula we give an alternative version of Fourier spectrum characterization for functions in the boundary Hardy spaces $H^p$ for $0<p\le 1.$

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• Fourier Spectrum Characterizations of Clifford $H^p$ Spaces on ${\mathbf{R}}_{+}^{n+1}$ for $1\le p\le \infty$