a(n) = 9 + floor( Sum_{j=1..n-1} a(j)/3 ).

Terms

a(0) =9a(1) =12a(2) =16a(3) =21a(4) =28a(5) =37a(6) =50a(7) =66a(8) =88a(9) =118a(10) =157a(11) =209a(12) =279a(13) =372a(14) =496a(15) =661a(16) =882a(17) =1176a(18) =1568a(19) =2090a(20) =2787a(21) =3716a(22) =4955a(23) =6606a(24) =8808a(25) =11744a(26) =15659a(27) =20879a(28) =27838a(29) =37118