a(n) = Sum_{k=0..n} binomial(2*n+1,6*k).

Terms

a(0) =1a(1) =1a(2) =1a(3) =8a(4) =85a(5) =463a(6) =1730a(7) =5461a(8) =18565a(9) =77540a(10) =349525a(11) =1486675a(12) =5858126a(13) =22369621a(14) =87087001a(15) =350739488a(16) =1431655765a(17) =5791193143a(18) =23100202490a(19) =91625968981a(20) =364760483725