16986931200domain: NAppears in sequencesa(n) = phi(n!!) where phi is the Euler totient function. In other words, a(n) = A000010(A006882(n)).at n=21A129335Denominators of coefficients of the series of sqrt(-log(1-q)/q).at n=8A272109a(n) is twice the square of the product of the first n primes each decreased by one.at n=6A307124