Finite searches, Chowla's cosine problem, and large Newman polynomials

Abstract: A length $n$ cosine sum is an expression of the form $\cos a_1\theta + \cdots + \cos a_n\theta$ where $a_1 < \cdots < a_n$ are positive integers, and a length $n$ Newman polynomial is an expression of the form $z^{a_1} + \cdots + z^{a_n}$ where $a_1 < \cdots < a_n$ are nonnegative integers. We define $-\lambda(n)$ to be the largest minimum of a length $n$ cosine sum as ${a_1,\ldots,a_n}$ ranges over all sets of $n$ positive integers, and we define $\mu(n)$ to be the largest minimum modulus on the unit circle of a length $n$ Newman polynomial as ${a_1,\ldots,a_n}$ ranges over all sets of $n$ nonnegative integers. Since there are infinitely many possibilities for the $a_j$, it is not obvious how to compute $\lambda(n)$ or $\mu(n)$ for a given $n$ in finitely many steps. Campbell et al. found the value of $\mu(3)$ in 1983, and Goddard found the value of $\mu(4)$ in 1992. In this paper, we find the values of $\lambda(2)$ and $\lambda(3)$ and nontrivial bounds on $\mu(5)$. We also include further remarks on the seemingly difficult general task of reducing the computation of $\lambda(n)$ or $\mu(n)$ to a finite problem.