Dennis Davenport | Searcharxiv

arXiv2025

Characterizing $ (\mathcal{F}, \mathcal{G}) $-syndetic, $ (\mathcal{F}, \mathcal{G}) $-thick, and related notions of size using derived sets along ultrafilters

We characterize relative notions of syndetic and thick sets using, what we call, "derived" sets along ultrafilters. Manipulations of derived sets is a characteristic feature of algebra in the Stone-Čech compactification and its applications. Combined with the existence of idempotents and structure of the smallest ideal in closed subsemigroups of the Stone-Čch compactification, our particular use of derived sets adapt and generalize methods recently used by Griffin arXiv:2311.09436 to characterize relative piecewise syndetic sets. As an application, we define an algebraically interesting subset of the Stone-Čech compactification and show, in some ways, it shares structural properties analogous to the smallest ideal.

math.GN↗

arXiv2013

Multiply partition regular matrices

Let $A$ be a finite matrix with rational entries. We say that $A$ is {\it doubly image partition regular\/} if whenever the set ${\mathbb N}$ of positive integers is finitely coloured, there exists $\vec x$ such that the entries of $A\vec x$ are all the same colour (or {\it monochromatic\/}) and also, the entries of $\vec x$ are monochromatic. Which matrices are doubly image partition regular? More generally, we say that a pair of matrices $(A,B)$, where $A$ and $B$ have the same number of rows, is {\it doubly kernel partition regular\/} if whenever ${\mathbb N}$ is finitely coloured, there exist vectors $\vec x$ and $\vec y$, each monochromatic, such that $A \vec x + B \vec y = 0$. There is an obvious sufficient condition for the pair $(A,B)$ to be doubly kernel partition regular, namely that there exists a positive rational $c$ such that the matrix $M=(\begin{array}{ccccc}A&cB\end{array})$ is kernel partition regular. (That is, whenever ${\mathbb N}$ is finitely coloured, there exists monochromatic $\vec x$ such that $M \vec x=\vec 0$.) Our aim in this paper is to show that this sufficient condition is also necessary. As a consequence we have that a matrix $A$ is doubly image partition regular if and only if there is a positive rational $c$ such that the matrix $(\begin{array}{lr}A&cI\end{array})$ is kernel partition regular, where $I$ is the identity matrix of the appropriate size. We also prove extensions to the case of several matrices.

math.CO↗