300180111058domain: NAppears in sequencesa(n) = Sum_{i=0..n} (n!/(n-i)!)^2.at n=9A006040a(2n-1) = n*a(2n-2), a(2n) = n*a(2n-1) + 1.at n=17A007876Interleaving A006040 and A228229.at n=17A375231