Improved lower bound for the number of unimodular zeros of self-reciprocal polynomials with coefficients in a finite set

Abstract

Let $n_1 < n_2 < \cdots < n_N$ be non-negative integers. In a private communication Brian Conrey asked how fast the number of real zeros of the trigonometric polynomials $T_N(\theta) = \sum_{j=1}^N {\cos (n_j\theta)}$ tends to $\infty$ as a function of $N$. Conrey's question in general does not appear to be easy.Let ${\mathcal P}_n(S)$ be the set of all algebraic polynomials of degree at most $n$ with each of their coefficients in $S$. For a finite set $S \subset {\mathbb C}$ let $M = M(S) := \max\{|z|: z \in S\}$. It has been shown recently that if $S \subset {\mathbb R}$ is a finite set and $(P_n)$ is a sequence of self-reciprocal polynomials $P_n \in {\mathcal P}_n(S)$ with $|P_n(1)|$ tending to $\infty$, then the number of zeros of $P_n$ on the unit circle also tends to $\infty$. In this paper we show that if $S \subset {\mathbb Z}$ is a finite set, then every self-reciprocal polynomial $P \in {\mathcal P}_n(S)$ has at least $$c(\log\log\log|P(1)|)^{1-\varepsilon}-1$$ zeros on the unit circle of ${\mathbb C}$ with a constant $c > 0$ depending only on $\varepsilon > 0$ and$M = M(S)$. Our new result improves the exponent $1/2 - \varepsilon$ in a recent result by Julian Sahasrabudhe to $1 - \varepsilon$. Sahasrabudhe's new idea [66] is combined with the approach used in [34] offering an essencially simplified way to achieve our improvement. We note that in both Sahasrabudhe's paper and our paper the assumption that the finite set $S$ contains only integers is deeply exploited.