Binomial coefficient C(3n,n-6).

Terms

a(0) =1a(1) =21a(2) =276a(3) =2925a(4) =27405a(5) =237336a(6) =1947792a(7) =15380937a(8) =118030185a(9) =886163135a(10) =6540715896a(11) =47626016970a(12) =343006888770