a(n) = (1/2)*(s(n) - s(n-1)), where s = A026180.

Terms

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