 Følner sequence

A Følner sequence is a sequence of nonempty finite subsets $F_n\subseteq G$ such that $|g{F}_n△F_n|/|F_n|\to 0$ for every $g\in G$.

A Følner sequence is a concept from the field of mathematical analysis, particularly in ergodic theory and group theory. It is named after the mathematician Ernst Følner. A Følner sequence provides a way to study the asymptotic behavior of actions of groups on sets and is often used in the context of amenable groups.