a(n) = floor(7^n/4^n).

Terms

a(0) =1a(1) =1a(2) =3a(3) =5a(4) =9a(5) =16a(6) =28a(7) =50a(8) =87a(9) =153a(10) =269a(11) =471a(12) =825a(13) =1443a(14) =2526a(15) =4421a(16) =7737a(17) =13540a(18) =23696a(19) =41468a(20) =72570a(21) =126998a(22) =222247a(23) =388933a(24) =680633a(25) =1191108a(26) =2084439a(27) =3647768a(28) =6383595a(29) =11171292