Littlewood polynomial

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}}\leq |p_{n}(z)|\leq 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) that satisfies the lower bound.

References

Peter Borwein (2002). Computational Excursions in Analysis and Number Theory. CMS Books in Mathematics. Springer-Verlag. pp. 2–5, 121–132. ISBN 0-387-95444-9.
J.E. Littlewood (1968). Some problems in real and complex analysis. D.C. Heath.