a(n) = Sum_{k=0..n} binomial(2*n,3*k).

Terms

a(0) =1a(1) =1a(2) =5a(3) =22a(4) =85a(5) =341a(6) =1366a(7) =5461a(8) =21845a(9) =87382a(10) =349525a(11) =1398101a(12) =5592406a(13) =22369621a(14) =89478485a(15) =357913942a(16) =1431655765a(17) =5726623061a(18) =22906492246a(19) =91625968981a(20) =366503875925