English

Stabilization in $H^\infty_{\mathbb{R}}(\mathbb{D})$

Complex Variables 2010-10-19 v2 Classical Analysis and ODEs

Abstract

In this paper we prove the following theorem: Suppose that f1,f2HR(\D)f_1,f_2\in H^\infty_\R(\D), with \normf1,\normf21\norm{f_1}_\infty,\norm{f_2}_{\infty}\leq 1, with infz\D(\absf1(z)+\absf2(z))=δ>0. \inf_{z\in\D}(\abs{f_1(z)}+\abs{f_2(z)})=\delta>0. Assume for some ϵ>0\epsilon>0 and small, f1f_1 is positive on the set of x(1,1)x\in(-1,1) where \absf2(x)<ϵ\abs{f_2(x)}<\epsilon for some ϵ>0\epsilon>0 sufficiently small. Then there exists g1,g11,g2HR(\D)g_1, g_1^{-1}, g_2\in H^\infty_\R(\D) with \normg1,\normg2,\normg11C(δ,ϵ) \norm{g_1}_\infty,\norm{g_2}_\infty,\norm{g_1^{-1}}_\infty\leq C(\delta,\epsilon) and f1(z)g1(z)+f2(z)g2(z)=1z\D. f_1(z)g_1(z)+f_2(z)g_2(z)=1\quad\forall z\in\D.

Keywords

Cite

@article{arxiv.0809.1573,
  title  = {Stabilization in $H^\infty_{\mathbb{R}}(\mathbb{D})$},
  author = {Brett D. Wick},
  journal= {arXiv preprint arXiv:0809.1573},
  year   = {2010}
}

Comments

v1: 22 pages, 2 figures, to appear in Pub. Mat; v2: 32 pages, 5 figures. The earlier version incorrectly claimed a characterization, as was pointed out by R. Mortini. A key hypothesis was strengthened with the main result remaining the same

R2 v1 2026-06-21T11:18:23.492Z