In number theory, the von Staudt-Clausen theorem is a result on the fractional part of Bernoulli numbers. Specifically, if we add 1/p to Bn for every prime p such that p − 1 divides n, we obtain an integer.
This fact immediately allows us to characterize the denominators of the non-zero Bernoulli numbers Bn as the product of all primes p such that p − 1 divides n; consequently the denominators are square-free and divisible by 6.
The result is named for Karl von Staudt (1798-1867) and Thomas Clausen (1801-1885), who independently discovered the result in 1840.