a(n) = floor(9^n/5^n).

Terms

a(0) =1a(1) =1a(2) =3a(3) =5a(4) =10a(5) =18a(6) =34a(7) =61a(8) =110a(9) =198a(10) =357a(11) =642a(12) =1156a(13) =2082a(14) =3748a(15) =6746a(16) =12143a(17) =21859a(18) =39346a(19) =70823a(20) =127482a(21) =229468a(22) =413042a(23) =743477a(24) =1338258a(25) =2408865a(26) =4335958a(27) =7804725a(28) =14048506a(29) =25287310