a(n) = Sum_{k=0..floor(3*n/8)} binomial(3*n-7*k,k).

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =6a(5) =9a(6) =18a(7) =36a(8) =64a(9) =119a(10) =228a(11) =423a(12) =786a(13) =1479a(14) =2766a(15) =5160a(16) =9661a(17) =18078a(18) =33786a(19) =63192a(20) =118212a(21) =221049a(22) =413391a(23) =773187a(24) =1445995a(25) =2704246a(26) =5057593a(27) =9458770a(28) =17689720a(29) =33083548