Numbers whose base-10 representation has the same nonzero number of 1's and 2's.

Terms

a(0) =12a(1) =21a(2) =102a(3) =120a(4) =123a(5) =124a(6) =125a(7) =126a(8) =127a(9) =128a(10) =129a(11) =132a(12) =142a(13) =152a(14) =162a(15) =172a(16) =182a(17) =192a(18) =201a(19) =210a(20) =213a(21) =214a(22) =215a(23) =216a(24) =217a(25) =218a(26) =219a(27) =231a(28) =241a(29) =251