Littlewood polynomial

From Wikipedia, the free encyclopedia
Jump to: navigation, search
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.


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 pn , 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.