1, 3 and the nonnegative even numbers.

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =4a(5) =6a(6) =8a(7) =10a(8) =12a(9) =14a(10) =16a(11) =18a(12) =20a(13) =22a(14) =24a(15) =26a(16) =28a(17) =30a(18) =32a(19) =34a(20) =36a(21) =38a(22) =40a(23) =42a(24) =44a(25) =46a(26) =48a(27) =50a(28) =52a(29) =54