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

Terms

a(0) =4a(1) =6a(2) =9a(3) =13a(4) =20a(5) =30a(6) =45a(7) =67a(8) =101a(9) =151a(10) =227a(11) =340a(12) =510a(13) =765a(14) =1148a(15) =1722a(16) =2583a(17) =3874a(18) =5811a(19) =8717a(20) =13075a(21) =19613a(22) =29419a(23) =44129a(24) =66193a(25) =99290a(26) =148935a(27) =223402a(28) =335103a(29) =502655