a(n) = s(3n) - s(3n-1), where s( ) is sequence A023635.

Terms

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