107112domain: NAppears in sequencesa(n) = Sum_{i=0..floor(n/2)} A047080(n,i).at n=20A047082a(n) = Sum_{i=0..floor((n+1)/2)} A047080(n,i).at n=20A047083Least number 2k such that p^2 divides the numerator of the Bernoulli number B(2k), where p is the n-th irregular prime, A000928(n).at n=31A092681