Numbers whose base-10 representation has the same nonzero number of 1's and 3's.

Terms

a(0) =13a(1) =31a(2) =103a(3) =123a(4) =130a(5) =132a(6) =134a(7) =135a(8) =136a(9) =137a(10) =138a(11) =139a(12) =143a(13) =153a(14) =163a(15) =173a(16) =183a(17) =193a(18) =213a(19) =231a(20) =301a(21) =310a(22) =312a(23) =314a(24) =315a(25) =316a(26) =317a(27) =318a(28) =319a(29) =321