In mathematics, specifically group theory, Frobenius's theorem states that if n divides the order of a finite group G, then the number of solutions of is a multiple of n. It was introduced by .
Related is Frobenius's conjecture (since proved, but not by Frobenius), which states that if the preceding is true, and the number of solutions of equals n, then the solutions form a normal subgroup.
A more general version of Frobenius's theorem states that if C is a conjugacy class with h elements of a finite group G with g elements and n is a positive integer, then the number of elements k such that k n is in C is a multiple of the greatest common divisor (hn,g) .
One application of Frobenius's theorem is to show that the coefficients of the Artin–Hasse exponential are p integral, by interpreting them in terms of the number of elements of order a power of p in the symmetric group Sn.
Frobenius conjectured that if in addition the number of solutions to is exactly n where n divides the order of G then these solutions form a normal subgroup. This has been proved as a consequence of the classification of finite simple groups.
The symmetric group S3 has exactly 4 solutions to but these do not form a normal subgroup; this is not a counterexample to the conjecture as 4 does not divide the order of S3 which is 6.