a(n) = 9 + floor((3 + Sum_{j=1..n-1} a(j))/4).

Terms

a(0) =9a(1) =12a(2) =15a(3) =18a(4) =23a(5) =29a(6) =36a(7) =45a(8) =56a(9) =70a(10) =88a(11) =110a(12) =137a(13) =171a(14) =214a(15) =268a(16) =335a(17) =418a(18) =523a(19) =654a(20) =817a(21) =1021a(22) =1277a(23) =1596a(24) =1995a(25) =2494a(26) =3117a(27) =3896a(28) =4870a(29) =6088