Improved Two-Colour Rado Numbers for Linear Equations with Certain Coefficients

Abstract

Let $a_1,\ldots,a_m$ be nonzero integers, $c \in \mathbb{Z}$ and $r \ge 2$. The Rado number for the equation
\[ \sum_{i=1}^m a_ix_i = c \]
in $r$ colours is the least positive integer $N$ such that any $r$-colouring of the integers in the interval $[1,N]$ admits a monochromatic solution to the given equation. We introduce the concept of $t$-distributability of sets of positive integers, and determine exact values whenever possible, and upper and lower bounds otherwise, for the Rado numbers when the set $\{a_1,\ldots,a_{m-1}\}$ is $2$-distributable or $3$-distributable, $a_m=-1$, and $r=2$. This generalizes previous works by several authors.