a(n) = (n!+6)/6.

Terms

a(0) =2a(1) =5a(2) =21a(3) =121a(4) =841a(5) =6721a(6) =60481a(7) =604801a(8) =6652801a(9) =79833601a(10) =1037836801a(11) =14529715201a(12) =217945728001