a(n) = n minus (number of 1's in binary expansion of n). Also highest power of 2 dividing n!.

Terms

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