Extensions of the Bloch-P\'olya theorem on the number of real zeros of polynomials (II)

Abstract

We prove that there is an absolute constant $c > 0$ such that for every $$a_0,a_1, \ldots,a_n \in [1,M]\,, \qquad 1 \leq M \leq \frac 14 \exp \left( \frac n9 \right)\,,$$ there are $$b_0,b_1,\ldots,b_n \in \{-1,0,1\}$$ such that the polynomial $P$ of the form $\displaystyle{P(z) = \sum_{j=0}^n{b_ja_jz^j}}$ has at least $\displaystyle{c \left( \frac{n}{\log(4M)} \right)^{1/2}-1}$ distinct sign changes in $I_a := (1-2a,1-a)$, where $\displaystyle{a := \left( \frac{\log(4M)}{n} \right)^{1/2} \leq 1/3}$. This improves and extends earlier results of Bloch and P\'olya and Erd\'elyi and, as a special case, recaptures a special case of a more general recent result of Jacob and Nazarov.