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The present paper establishes a complete result on approximation by rational functions with prescribed numerator degree in Lpspaces for 1 < p < ∞ and proves that if f(x)∈Lp[-1,1] changes sign exactly l times in (-1,1), then there exists r(x)∈Rnl such that

\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{bbm} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\left\| {f(x) - r(x)} \right\|_{L^p } \leqq C_{p,l,b} \omega (f,n^{ - 1} )_{L^p } ,$$ \end{document}
where Rnl indicates all rational functions whose denominators consist of polynomials of degree n and numerators polynomials of degree l, and Cp, l,b is a positive constant depending only on p, l and b which relates to the distance among the sign change points of f(x) and will be given in 3.

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