In number theory, the von Staudt–Clausen theorem is a result determining the fractional part of Bernoulli numbers, found independently by Karl von Staudt (1840) and Thomas Clausen (1840).
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.
These denominators are
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The von Staudt–Clausen theorem has two parts. The first one describes how the denominators of the Bernoulli numbers can be computed. Paraphrasing the words of Clausen it can be stated as:
“The denominator of the 2nth Bernoulli number can be found as follows: Add to all divisors of 2n, 1, 2, a, a', ..., 2n the unity, which gives the sequence 2, 3, a + 1, a' + 1, ..., 2n + 1. Select from this sequence only the prime numbers 2, 3, p, p', etc. and build their product.”
The second part of the von Staudt–Clausen theorem is a representation of the Bernoulli numbers. This representation is given for the first few nonzero Bernoulli numbers in the next table.
| Von Staudt–Clausen representation of Bn | ||
| B0 | = | 1 |
| B1 | = | − 1/2 |
| B2 | = | 1 − 1/2 − 1/3 |
| B4 | = | 1 − 1/2 − 1/3 − 1/5 |
| B6 | = | 1 − 1/2 − 1/3 − 1/7 |
| B8 | = | 1 − 1/2 − 1/3 − 1/5 |
| B10 | = | 1 − 1/2 − 1/3 − 1/11 |