Littlewood polynomial

For the orthogonal polynomials in several variables, see Hall–Littlewood polynomials.

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 $a_i = \pm 1$. Littlewood's problem asks for constants c₁ and c₂ such that there are infinitely many Littlewood polynomials p_n , of increasing degree n satisfying

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

for all $z$ on the unit circle. The Rudin-Shapiro polynomials provide a sequence satisfying the upper bound with $c_2 = \sqrt 2$. No sequence is known (as of 2008) which satisfies the lower bound.