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

Terms

a(0) =1a(1) =7a(2) =46a(3) =299a(4) =1941a(5) =12616a(6) =82160a(7) =536155a(8) =3505699a(9) =22964087a(10) =150676186a(11) =990134948a(12) =6515349244a(13) =42925973608a(14) =283134975936