Binomial coefficient C(3n,n-4).

Terms

a(0) =1a(1) =15a(2) =153a(3) =1330a(4) =10626a(5) =80730a(6) =593775a(7) =4272048a(8) =30260340a(9) =211915132a(10) =1471442973a(11) =10150595910a(12) =69668534468a(13) =476260169700