Approximation by power series with $\pm 1$ coefficients

Abstract

In this paper we construct certain type of near-optimal approximations of a class of analytic functions in the unit disc by power series with two distinct coefficients. More precisely, we show that if all the coefficients of the power series f(z) are real and lie in [-a,a] where a < 1, then there exists a power series Q(z) with coefficients in {-1,+1} such that |f(z)-Q(z)| approaches 0 at the rate exp(-C/|1-z|) as z approaches 1 non-tangentially inside the unit disc. A result by Borwein-Erdelyi-Kos shows that this type of decay rate is best possible. The special case f=0 yields a near-optimal solution to the "fair duel" problem of Konyagin.