Numbers with exactly 3 1's in their base 4 expansion.

Terms

a(0) =21a(1) =69a(2) =81a(3) =84a(4) =86a(5) =87a(6) =89a(7) =93a(8) =101a(9) =117a(10) =149a(11) =213a(12) =261a(13) =273a(14) =276a(15) =278a(16) =279a(17) =281a(18) =285a(19) =293a(20) =309a(21) =321a(22) =324a(23) =326a(24) =327a(25) =329a(26) =333a(27) =336a(28) =338a(29) =339