Source: wikibot/ping-pong-lemma
= Ping-pong lemma
{wiki=Ping-pong_lemma}
The Ping-Pong Lemma is a result in geometric group theory that is often used to prove that a group is free, or to show that a group has a particular property, such as being non-abelian or having a certain type of subgroup. The lemma is particularly useful in the context of groups acting on trees or hyperbolic spaces.