English

Unbounded Widom factors for orthogonal and residual polynomials

Complex Variables 2025-08-22 v1 Classical Analysis and ODEs

Abstract

We study Widom factors for (a) monic orthogonal polynomials in L2L^2 with respect to the equilibrium measure of a compact set KRK\subset\mathbb{R} and (b) residual polynomials normalized at an exterior point. Using weakly equilibrium Cantor sets K(γ)K(\gamma), we prove: (1) Given any sequence (cn)(c_n) with subexponential growth, there exists a non-polar Cantor set K(γ)K(\gamma), depending on (cn)(c_n), such that the L2L^2 Widom factors of the associated orthogonal polynomials (with respect to the equilibrium measure of K(γ)K(\gamma)) exceed cnc_n for every nn. (2) For the same K(γ)K(\gamma) built from (cn)(c_n) and each exterior point x0RK(γ)x_0\in\mathbb{R}\setminus K(\gamma), the residual Widom factors satisfy power-type lower bounds with a Harnack-distance exponent τx0(0,1)\tau_{x_0}\in(0,1): they are bounded below by cnτx0c_n^{\tau_{x_0}} for all degrees when x0x_0 lies in an unbounded gap, and along a subsequence of degrees when x0x_0 lies in a bounded gap. Consequently, if, in addition, (cn)(c_n) is monotone increasing and unbounded, then the sequence of residual Widom factors is unbounded for every x0RK(γ)x_0\in\mathbb{R}\setminus K(\gamma). The proofs combine inverse-image constructions, capacity comparisons for period-nn sets, harmonic-measure representations for differences of Green functions, and alternation principles on nested approximants.

Keywords

Cite

@article{arxiv.2508.15131,
  title  = {Unbounded Widom factors for orthogonal and residual polynomials},
  author = {Gökalp Alpan},
  journal= {arXiv preprint arXiv:2508.15131},
  year   = {2025}
}