The natural numbers interleaved with the even numbers.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =2a(5) =4a(6) =3a(7) =6a(8) =4a(9) =8a(10) =5a(11) =10a(12) =6a(13) =12a(14) =7a(15) =14a(16) =8a(17) =16a(18) =9a(19) =18a(20) =10a(21) =20a(22) =11a(23) =22a(24) =12a(25) =24a(26) =13a(27) =26a(28) =14a(29) =28