Optimization tools for computing colorings of [1,…,n] with few monochromatic solutions on 3-variable linear equations

Abstract

A famous result in arithmetic Ramsey theory says that for many linear homogeneous equations $E$ there is a threshold value $R_k\left(E\right)$ (the Rado number of $E$) such that for any $k$-coloring of the integers in the interval $[1,n]$, with $n\ge R_k\left(E\right)$, there exists at least one monochromatic solution. But one can further ask, how many monochromatic solutions is the minimum possible in terms of $n$? Several authors have estimated this function before, here we offer new tools from integer and semidefinite optimization that help find either optimal or near optimal 2-colorings minimizing the number of monochromatic solutions of several families of 3-variable non-regular homogeneous linear equations. In the last part of the paper we further extend to three and more colors for the Schur equation, improving earlier work.