Group Marriage Problem

TL;DR

This paper investigates conditions under which permutation groups admit a structured matching, extending classical marriage theorems to broader group actions and identifying precise group structures that guarantee solutions.

Contribution

It characterizes when the orbit condition guarantees a G-marriage, showing it holds only for direct products of symmetric groups, and extends the orbit condition concept to k-orbit conditions for specific groups.

Findings

Orbit condition is sufficient iff G is a direct product of symmetric groups.

Extended the orbit condition to k-orbit condition for certain groups.

Established equivalence between (n-1)-orbit condition and G-marriage for alternating and cyclic groups.

Abstract

Let $G$ be a permutation group acting on $[n]=\{1, ..., n\}$ and $\mathcal{V}=\{V_{i}: i=1, ..., n\}$ be a system of $n$ subsets of $[n]$. When is there an element $g \in G$ so that $g(i) \in V_{i}$ for each $i \in [n]$? If such $g$ exists, we say that $G$ has a $G$-marriage subject to $\mathcal{V}$. An obvious necessary condition is the {\it orbit condition}: for any $\emptyset \not = Y \subseteq [n]$, $\bigcup_{y \in Y} V_{y} \supseteq Y^{g}=\{g(y): y \in Y \}$ for some $g \in G$. Keevash (J. Combin. Theory Ser. A 111(2005), 289--309) observed that the orbit condition is sufficient when $G$ is the symmetric group $\Sym([n])$; this is in fact equivalent to the celebrated Hall's Marriage Theorem. We prove that the orbit condition is sufficient if and only if $G$ is a direct product of symmetric groups. We extend the notion of orbit condition to that of $k$-orbit condition and prove that if $G$ is the alternating group $\Alt([n])$ or the cyclic group $C_{n}$ where $n \ge 4$, then $G$ satisfies the $(n-1)$-orbit condition subject to $\V$ if and only if $G$ has a $G$-marriage subject to $\mathcal{V}$.