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

Terms

a(0) =1a(1) =5a(2) =26a(3) =146a(4) =861a(5) =5229a(6) =32361a(7) =202905a(8) =1284480a(9) =8191380a(10) =52543545a(11) =338641305a(12) =2191124301a(13) =14224347181a(14) =92603307541a(15) =604342068085