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

Terms

a(0) =1a(1) =1a(2) =2a(3) =21a(4) =86a(5) =305a(6) =1381a(7) =6042a(8) =24901a(9) =105364a(10) =450017a(11) =1902321a(12) =8044478a(13) =34114553a(14) =144535714a(15) =612041073a(16) =2592800365a(17) =10984222558a(18) =46527959417a(19) =197093207976a(20) =834913179137