On Regular Set Systems Containing Regular Subsystems

Abstract: Let $X,Y$ be finite sets, $r,s,h, \lambda \in \mathbb{N}$ with $s\geq r, X\subsetneq Y$. By $\lambda \binom{X}{h}$ we mean the collection of all $h$-subsets of $X$ where each subset occurs $\lambda$ times. A coloring of $\lambda\binom{X}{h}$ is {\it $r$-regular} if in every color class each element of $X$ occurs $r$ times. A one-regular color class is a {\it perfect matching}. We are interested in the necessary and sufficient conditions under which an $r$-regular coloring of $\lambda \binom{X}{h}$ can be embedded into an $s$-regular coloring of $\lambda \binom{Y}{h}$. Using algebraic techniques involving glueing together orbits of a suitably chosen cyclic group, the first author and Newman (Combinatorica 38 (2018), no. 6, 1309--1335) solved the case when $\lambda=1,r=s, \gcd (|X|,|Y|,h)=\gcd(|Y|,h)$. Using purely combinatorial techniques, we nearly settle the case $h=4$. Two major challenges include finding all the necessary conditions, and obtaining the exact bound for $|Y|$. It is worth noting that completing partial symmetric latin squares is closely related to the case $\lambda =r=s=1, h=2$ which was solved by Cruse (J. Comb. Theory Ser. A 16 (1974), 18--22).