a(2) = 1, a(3)=3; for n >= 4, a(n) = (n-2)!*Stirling_2(n,n-1)/2 = n!/4.

A133799

a(2) = 1, a(3)=3; for n >= 4, a(n) = (n-2)!*Stirling_2(n,n-1)/2 = n!/4.

Terms

    a(0) =1a(1) =3a(2) =6a(3) =30a(4) =180a(5) =1260a(6) =10080a(7) =90720a(8) =907200a(9) =9979200a(10) =119750400a(11) =1556755200a(12) =21794572800a(13) =326918592000

External references