Paper detail

Arguments of zeros of highly log concave polynomials

For a real polynomial $p = \sum_{i=0}^{n} c_ix^i$ with no negative coefficients and $n\geq 6$, let $β(p) = \inf_{i=1}^{n-1} c_i^2/c_{i+1}c_{i-1}$ (so $β(p) \geq 1$ entails that $p$ is log concave). If $β(p) > 1.45...$, then all roots of $p$ are in the left half plane, and moreover, there is a function $β_0 (θ)$ (for $π/2 \leq θ\leq π$) \st $β\geq β_0(θ)$ entails all roots of $p$ have arguments in the sector $| \arg z| \geq θ$ with the smallest possible $θ$; 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 $β\geq 4$ entails all roots are real). We also prove a version of Kurtz's theorem with real (not necessarily nonnegative) coefficients.