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

Terms

a(0) =1a(1) =4a(2) =26a(3) =181a(4) =1252a(5) =8657a(6) =59864a(7) =413966a(8) =2862617a(9) =19795288a(10) =136886433a(11) =946583628a(12) =6545722210a(13) =45264335853a(14) =313007493212