The dimension of any irreducible representation of a group must be a divisor of the index of each maximal normal Abelian subgroup of .

Note that while Itô's theorem was proved by Noboru Itô, Ito's lemma was proven by Kiyoshi Ito.

More things to try:

Weisstein, Eric W. "Itô's Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ItosTheorem.html