Numbers whose base-4 representation has the same nonzero number of 1's and 2's.

Terms

a(0) =6a(1) =9a(2) =18a(3) =24a(4) =27a(5) =30a(6) =33a(7) =36a(8) =39a(9) =45a(10) =54a(11) =57a(12) =66a(13) =72a(14) =75a(15) =78a(16) =90a(17) =96a(18) =99a(19) =102a(20) =105a(21) =108a(22) =111a(23) =114a(24) =120a(25) =123a(26) =126a(27) =129a(28) =132a(29) =135