Binomial coefficient C(3n, n-3).

Terms

a(0) =1a(1) =12a(2) =105a(3) =816a(4) =5985a(5) =42504a(6) =296010a(7) =2035800a(8) =13884156a(9) =94143280a(10) =635745396a(11) =4280561376a(12) =28760021745a(13) =192928249296