a(n) = round(3*(4/3)^n).

Terms

a(0) =3a(1) =4a(2) =5a(3) =7a(4) =9a(5) =13a(6) =17a(7) =22a(8) =30a(9) =40a(10) =53a(11) =71a(12) =95a(13) =126a(14) =168a(15) =224a(16) =299a(17) =399a(18) =532a(19) =710a(20) =946a(21) =1261a(22) =1682a(23) =2242a(24) =2990a(25) =3986a(26) =5315a(27) =7087a(28) =9449a(29) =12599