Monochromatic sumsets in countable colourings of abelian groups
Volume 72 / 2024
Bulletin Polish Acad. Sci. Math. 72 (2024), 97-102
MSC: Primary 05D10
DOI: 10.4064/ba240807-24-11
Published online: 18 December 2024
Abstract
Fernández-Bretón, Sarmiento and Vera showed that whenever a direct sum of sufficiently many copies of $\mathbb Z_4$, the cyclic group of order 4, is countably coloured there are arbitrarily large finite sets $X$ whose sumsets $X+X$ are monochromatic. They asked if the elements of order 4 are necessary, in the following strong sense: if $G$ is an abelian group having no elements of order 4, is it always the case that there is a countable colouring of $G$ for which there is not even a monochromatic sumset $X+X$ with $X$ of size 2? Our aim in this short note is to show that this is indeed the case.
