61664980320domain: NAppears in sequencesa(n) = H(2*n)*(2*n)!/n! where H(n) = Sum_{k=1..n} 1/k, the n-th harmonic number.at n=9A123989a(n) = H(n)*n!/(floor(n/2))!, where H(n) = Sum_{k=1..n} 1/k, the n-th harmonic number.at n=18A124078