a(n) = floor(7^n/2^n).

Terms

a(0) =1a(1) =3a(2) =12a(3) =42a(4) =150a(5) =525a(6) =1838a(7) =6433a(8) =22518a(9) =78815a(10) =275854a(11) =965491a(12) =3379220a(13) =11827271a(14) =41395451a(15) =144884079a(16) =507094277a(17) =1774829971a(18) =6211904899a(19) =21741667147a(20) =76095835015a(21) =266335422555a(22) =932173978944