a(n) = 12 + floor((1 + Sum_{j=1..n-1} a(j))/4).

Terms

a(0) =12a(1) =15a(2) =19a(3) =23a(4) =29a(5) =36a(6) =45a(7) =57a(8) =71a(9) =89a(10) =111a(11) =139a(12) =173a(13) =217a(14) =271a(15) =339a(16) =423a(17) =529a(18) =661a(19) =827a(20) =1033a(21) =1292a(22) =1615a(23) =2018a(24) =2523a(25) =3154a(26) =3942a(27) =4928a(28) =6160a(29) =7700