On the oscillations of the modulus of Rudin-Shapiro polynomials around the middle of their ranges

Abstract

Let either $R_k(t) := |P_k(e^{it})|^2$ or $R_k(t) := |Q_k(e^{it})|^2$, where $P_k$ and $Q_k$ are the usual Rudin-Shapiro polynomials of degree $n-1$ with $n=2^k$. The graphs of the trigonometric polynomials $R_k$ on the period suggest many zeros of $R_k(t)-n$ in a dense fashion on the period. Let ${\Cal N}(I,R_k-n)$ denote the number of zeros, counted with multiplicities, of the trigonometric polynomial $R_k-n$ in an interval $I := [\alpha,\beta] \subset [0,2\pi)$. Improving earlier results proved only for the interval $I := [0,2\pi)$, in this paper we show that $$\frac{n|I|}{8\pi} - \frac{2}{\pi} (2n\log n)^{1/2} - 1 \leq N(I,R_k-n) \leq \frac{n|I|}{\pi} + \frac{8}{\pi}(2n\log n)^{1/2}\,,\qquad k \geq 2\,,$$ for every interval $I := [\alpha,\beta] \subset [0,2\pi)$, where $|I| = \beta-\alpha$ denotes the length of the interval $I$.