Littlewood polynomial

File:Roots of Littlewood polynomials with degree 15.png

Roots of all the Littlewood polynomials of degree 15.

File:LittlewoodPolynomialRootsDegree1-14.gif

An animation showing the roots of all Littlewood polynomials with degree 1 through 14, one degree at a time.

In mathematics, a Littlewood polynomial is a polynomial all of whose coefficients are +1 or −1. Littlewood's problem asks how large the values of such a polynomial must be on the unit circle in the complex plane. The answer to this would yield information about the autocorrelation of binary sequences. They are named for J. E. Littlewood who studied them in the 1950s.

Definition

A polynomial

$p(x)=\sum _{i=0}^n{a}_i{x}^i$

is a Littlewood polynomial if all the a_i = ±1. Littlewood's problem asks for constants c₁ and c₂ such that there are infinitely many Littlewood polynomials p_n, of increasing degree Template:Mvar satisfying

$c_1\sqrt{n+1}\le |p_n(z)|\le c_2\sqrt{n+1}.$

for all Template:Mvar on the unit circle. The Rudin–Shapiro polynomials provide a sequence satisfying the upper bound with c₂ = Template:Radical. In 2019, an infinite family of Littlewood polynomials satisfying both the upper and lower bound was constructed by Paul Balister, Béla Bollobás, Robert Morris, Julian Sahasrabudhe, and Marius Tiba.