a(n) = (s(n)-s(n-1))/2, where s = A026139.

Terms

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