a(n) = Sum_{k=0..n} binomial(2*n+1,3*k).

Terms

a(0) =1a(1) =2a(2) =11a(3) =43a(4) =170a(5) =683a(6) =2731a(7) =10922a(8) =43691a(9) =174763a(10) =699050a(11) =2796203a(12) =11184811a(13) =44739242a(14) =178956971a(15) =715827883a(16) =2863311530a(17) =11453246123a(18) =45812984491a(19) =183251937962a(20) =733007751851