Floor(((1+sqrt(7))/2)^n).

Terms

a(0) =1a(1) =1a(2) =3a(3) =6a(4) =11a(5) =20a(6) =36a(7) =66a(8) =121a(9) =222a(10) =405a(11) =738a(12) =1346a(13) =2453a(14) =4472a(15) =8153a(16) =14863a(17) =27093a(18) =49388a(19) =90029a(20) =164112a(21) =299156a(22) =545324a(23) =994058a(24) =1812045a(25) =3303133a(26) =6021201a(27) =10975901a(28) =20007704a(29) =36471557