Baernstein's quasi-norm monotonicity conjecture for polynomials with unimodular zero

Abstract

Let $m$ denote the normalized Haar measure on the unit circle $\mathbb{T}$. For $0<r<\infty$, define $\lVert f\rVert_{r}:=\left(\int_{\mathbb{T}}|f|^{r}\,\mathrm{d} m\right)^{1/r}$, with $\lVert f\rVert_{0}$ and $\lVert f\rVert_{\infty}$ interpreted as the geometric mean and the supremum norm, respectively. Set $Q_n(z)=1+z^n$. We prove that, for every nonzero polynomial $p$ of degree $n$ whose zeros all lie on $\mathbb{T}$, $$ \frac{\lVert p\rVert_s}{\lVert Q_n\rVert_s} \le \frac{\lVert p\rVert_t}{\lVert Q_n\rVert_t}, \qquad 0\le s\le t\le\infty. $$ This settles Baernstein&#39;s quasi-norm monotonicity conjecture. As corollaries, we obtain an $L^r$ extension of Visser&#39;s coefficient inequality, the sharp O&#39;Hara--Rodriguez inequality and its higher-power analogues, the Erdős--Szekeres product bound $ \left\lVert\prod_{j=1}^N(1-z^{s_j})\right\rVert_\infty\ge2\sqrt N $ for all positive integers $s_1,\ldots,s_N$ and Agler--McCarthy&#39;s entropy conjecture. We also provide a Lean 4 formalization of the main results.