# circle

## Chowla's cosine problem ★★★

Author(s): Chowla

\begin{problem} Let $A \subseteq {\mathbb N}$ be a set of $n$ positive integers and set $m(A) = - \min_x \sum_{a \in A} \cos(ax).$ What is $m(n) = \min_A m(A)$? \end{problem}

Keywords: circle; cosine polynomial