Improved results on the oscillation of the modulus of the Rudin-Shapiro polynomials on the unit circle

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 $n=2^k$. In a recent paper we combined close to sharp upper bounds for the modulus of the autocorrelation coefficients of the Rudin-Shapiro polynomials with a deep theorem of Littlewood to prove that there is an absolute constant $A>0$ such that the equation $R_k(t) = (1+\eta )n$ has at least $An^{0.5394282}$ distinct zeros in $[0,2\pi)$ whenever $\eta$ is real and $|\eta| < 2^{-11}$. In this paper we show that the equation $R_k(t)=(1+\eta)n$ has at least $(1/2-|\eta|-\varepsilon)n/2$ distinct zeros in $[0,2\pi)$ for every $\eta \in (-1/2,1/2)$, $\varepsilon > 0$, and sufficiently large $k \geq k_{\eta,\varepsilon}$.