Denominator from divisors
Denominators: a check without a table
For even n of 2 or more, the denominator of Bn is the product of the primes p
for which p − 1 divides n.
von Staudt-Clausen theorem [M4]
B12 in the table
−691
2730
2730
matches
Necessary, not sufficient: a wrong numerator over the right denominator passes.