1's-counting sequence: number of 1's in binary expansion of n (or the binary weight of n).

Terms

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