a(n) = Sum_{k=0..n} binomial(2*n,7*k).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =9a(5) =121a(6) =793a(7) =3434a(8) =11561a(9) =34885a(10) =116281a(11) =490337a(12) =2309385a(13) =10381281a(14) =42484682a(15) =161766061a(16) =603861897a(17) =2326683921a(18) =9402807817a(19) =38936059929a(20) =160095493289a(21) =644032289258