Herbrand-Ribet theorem

Etymology
Named after and.

Proper noun

 * 1)  A result on the class group of certain number fields, strengthening Ernst Kummer's theorem to the effect that the prime p divides the class number of the cyclotomic field of p-th roots of unity iff p divides the numerator of the n-th Bernoulli number Bn for some n, 0 < n < p &minus; 1. The Herbrand–Ribet theorem specifies what, in particular, it means when p divides such an Bn.