17465201775domain: NAppears in sequencesNumber of permutations in S_n avoiding the strings 123, 321 and 231.at n=22A060696Sum of product of odd numbers <= n and the product of even numbers <= n.at n=20A076051a(n) = (2n+1)!! + (2n)!! where k!! = A006882(k).at n=9A306185