In group theory, Hajós's theorem states that if a finite abelian group is expressed as the Cartesian product of simplexes, that is, sets of the form { e, a, a 2, …, a s − 1 } {\displaystyle \{e,a,a^{2},\dots,a^{s-1}\}} where e {\displaystyle e} is the identity element, then at least one of the factors is a subgroup. Wikipedia