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.
Itô's Theorem
See also
Abelian Group, Irreducible Representation, SubgroupExplore with Wolfram|Alpha
References
Itô, N. "On the Degrees of Irreducible Representations of a Finite Group." Nagoya Math. J. 3, 5-6, 1951.Lomont, J. S. Applications of Finite Groups. New York: Dover, p. 55, 1993.Referenced on Wolfram|Alpha
Itô's TheoremCite this as:
Weisstein, Eric W. "Itô's Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ItosTheorem.html