Numbers of the form 9*k+3 or 9*k+6.

Terms

a(0) =3a(1) =6a(2) =12a(3) =15a(4) =21a(5) =24a(6) =30a(7) =33a(8) =39a(9) =42a(10) =48a(11) =51a(12) =57a(13) =60a(14) =66a(15) =69a(16) =75a(17) =78a(18) =84a(19) =87a(20) =93a(21) =96a(22) =102a(23) =105a(24) =111a(25) =114a(26) =120a(27) =123a(28) =129a(29) =132