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On a common-extendable, non-Sidorenko linear system

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https://doi.org/10.5070/C63362788Creative Commons 'BY' version 4.0 license
Abstract

A system of linear equations in \(\mathbb{F}_p^n\) is common if every two-colouring of \(\mathbb{F}_p^n\) yields at least as many monochromatic solutions as a random two-colouring, asymptotically as \(n \to \infty\). By analogy to the graph-theoretic setting, Alon has asked whether any (non-Sidorenko) system of linear equations can be made uncommon by adding sufficiently many free variables. Fox, Pham and Zhao answered this question in the affirmative among systems which consist of a single equation. We answer Alon's question in the negative.

We also observe that the property of remaining common despite that addition of arbitrarily many free variables is closely related to a notion of commonness in which one replaces the arithmetic mean of the number of monochromatic solutions with the geometric mean, and furthermore resolve questions of Kamčev-Liebenau-Morrison.

Mathematics Subject Classifications: 05D10, 11B30

Keywords: Sidorenko's conjecture, Sidorenko and common linear patterns

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