Quadratic spectral concentration of characteristic functions

Abstract

It is known that the inequality \begin{align*}\int_{-W/2}^{W/2}|\widehat{f}(\xi)|^2d\xi\leq \int_{-W/2}^{W/2}|\widehat{|f|^*}(\xi)|^2d\xi \end{align*} between the quadratic spectral concentration of a function and that of its decreasing rearrangement holds for any function $f\in L^2,\;|\text{supp} f|=T,$ if and only if the product $WT$ does not exceed the critical value $\approx 0.884$. We show that by restricting ourselves to characteristic functions we can enlarge this range up to $WT\leq 4/3$. Besides, we establish various properties of minimizers of the difference $\int_{-W/2}^{W/2}|\widehat{\chi_A^*}(\xi)|^2d\xi-\int_{-W/2}^{W/2}|\widehat{\chi_A}(\xi)|^2d\xi$ over sets $A$ of finite measure and prove that this difference is non-negative for all $W,T>0$ if $A$ is the union of two intervals. As a corollary, we obtain a sharp (up to a constant) estimate for the $L_2$-norms of non-harmonic trigonometric polynomials with alternating coefficients $\pm 1$.