a(n) = 6 + floor( Sum_{j=1..n-1} a(j)/4 ).

Terms

a(0) =6a(1) =7a(2) =9a(3) =11a(4) =14a(5) =17a(6) =22a(7) =27a(8) =34a(9) =42a(10) =53a(11) =66a(12) =83a(13) =103a(14) =129a(15) =161a(16) =202a(17) =252a(18) =315a(19) =394a(20) =492a(21) =615a(22) =769a(23) =961a(24) =1202a(25) =1502a(26) =1878a(27) =2347a(28) =2934a(29) =3667