a(n) = (1/2)*|s(n) - s(n-1)|, where s = A026145.

Terms

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