a(n) = 8 + floor( (1 + Sum_{j=1..n-1} a(j)) / 2).

Terms

a(0) =8a(1) =12a(2) =18a(3) =27a(4) =41a(5) =61a(6) =92a(7) =138a(8) =207a(9) =310a(10) =465a(11) =698a(12) =1047a(13) =1570a(14) =2355a(15) =3533a(16) =5299a(17) =7949a(18) =11923a(19) =17885a(20) =26827a(21) =40241a(22) =60361a(23) =90542a(24) =135813a(25) =203719a(26) =305579a(27) =458368a(28) =687552a(29) =1031328