2, 4 and the odd numbers.

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =5a(5) =7a(6) =9a(7) =11a(8) =13a(9) =15a(10) =17a(11) =19a(12) =21a(13) =23a(14) =25a(15) =27a(16) =29a(17) =31a(18) =33a(19) =35a(20) =37a(21) =39a(22) =41a(23) =43a(24) =45a(25) =47a(26) =49a(27) =51a(28) =53a(29) =55