The $L_q$ norm of the Rudin-Shapiro polynomials on subarcs of the unit circle

Abstract

Littlewood polynomials are polynomials with each of their coefficients in $\{-1,1\}$. A sequence of Littlewood polynomials that satisfies a remarkable flatness property on the unit circle of the complex plane is given by the Rudin-Shapiro polynomials. Let $P_k$ and $Q_k$ denote the Rudin-Shapiro polynomials of degree $n-1$ with $n:=2^k$. For polynomials $S$ we define $$M_q(S,[\alpha,\beta]) := \left( \frac{1}{\beta-\alpha} \int_{\alpha}^{\beta} {\left| S(e^{it}) \right|^q\,dt} \right)^{1/q}\,, \qquad q > 0\,.$$ Let $\gamma := \sin^2(\pi/8)$. We prove that $$\frac{\gamma}{4\pi}(\gamma n)^{q/2} \leq M_q(P_k,[\alpha,\beta])^q \leq (2n)^{q/2}$$ for every $q > 0$ and $32\pi/n \leq \beta-\alpha$. The same estimates hold for $P_k$ replaced by $Q_k$.