a(n) = Sum_{k=0..n} binomial(3*n+1,6*k).

Terms

a(0) =1a(1) =1a(2) =8a(3) =211a(4) =1730a(5) =9829a(6) =77540a(7) =728575a(8) =5858126a(9) =43942081a(10) =350739488a(11) =2884834891a(12) =23100202490a(13) =182670807229