English

Arguments of zeros of highly log concave polynomials

Classical Analysis and ODEs 2010-10-01 v1

Abstract

For a real polynomial p=i=0ncixip = \sum_{i=0}^{n} c_ix^i with no negative coefficients and n6n\geq 6, let β(p)=infi=1n1ci2/ci+1ci1\beta (p) = \inf_{i=1}^{n-1} c_i^2/c_{i+1}c_{i-1} (so β(p)1\beta (p) \geq 1 entails that pp is log concave). If β(p)>1.45...\beta(p) > 1.45..., then all roots of pp are in the left half plane, and moreover, there is a function β0(θ)\beta_0 (\theta) (for π/2θπ\pi/2 \leq \theta \leq \pi) \st ββ0(θ)\beta \geq \beta_0(\theta) entails all roots of pp have arguments in the sector argzθ| \arg z| \geq \theta with the smallest possible θ\theta; we determine exactly what this function (and its inverse) is (it turns out to be piecewise smooth, and quite tractible). This is a one-parameter extension of Kurtz's theorem (which asserts that β4\beta \geq 4 entails all roots are real). We also prove a version of Kurtz's theorem with real (not necessarily nonnegative) coefficients.

Keywords

Cite

@article{arxiv.1009.6022,
  title  = {Arguments of zeros of highly log concave polynomials},
  author = {David Handelman},
  journal= {arXiv preprint arXiv:1009.6022},
  year   = {2010}
}