a(n) = 3 + floor((2 + Sum_{j=1..n-1} a(j))/4).

Terms

a(0) =3a(1) =4a(2) =5a(3) =6a(4) =8a(5) =10a(6) =12a(7) =15a(8) =19a(9) =24a(10) =30a(11) =37a(12) =46a(13) =58a(14) =72a(15) =90a(16) =113a(17) =141a(18) =176a(19) =220a(20) =275a(21) =344a(22) =430a(23) =538a(24) =672a(25) =840a(26) =1050a(27) =1313a(28) =1641a(29) =2051