4th-largest term in n-th row of Stern's diatomic triangle A002487.

Terms

a(0) =2a(1) =4a(2) =10a(3) =17a(4) =29a(5) =47a(6) =79a(7) =128a(8) =208a(9) =337a(10) =546a(11) =883a(12) =1429a(13) =2312a(14) =3741a(15) =6053a(16) =9794a(17) =15847a(18) =25641a(19) =41488a(20) =67129a(21) =108617