Uniform approximation of sgn(x) by rational functions with prescribed poles

Abstract

For $a\in (0,1)$ let $L_m^k(a)$ be the error of the best approximation of the function $\sgn(x)$ on the two symmetric intervals $[-1,-a]\cup [a,1]$ by rational functions with the only possible poles of degree $2k-1$ at the origin and of $2m-1$ at infinity. Then the following limit exists \begin{equation} \lim_{m\to \infty}L^k_m(a)(\frac{1+a}{1-a})^{m-{1/2}} (2m-1)^{k+{1/2}}=\frac 2 π(\frac{1-a^2}{2a})^{k+{1/2}} Γ(k+\frac 1 2). \end{equation}

Cited by in corpus (1)

• Asymptotics of the best polynomial approximation of $|x{|}^p$ and of the best Laurent polynomial approximation of $\sgn(x)$ on two symmetric intervals