Numbers that contain only 1's and 2's. Nonempty binary strings of length n in lexicographic order.

Terms

a(0) =1a(1) =2a(2) =11a(3) =12a(4) =21a(5) =22a(6) =111a(7) =112a(8) =121a(9) =122a(10) =211a(11) =212a(12) =221a(13) =222a(14) =1111a(15) =1112a(16) =1121a(17) =1122a(18) =1211a(19) =1212a(20) =1221a(21) =1222a(22) =2111a(23) =2112a(24) =2121a(25) =2122a(26) =2211a(27) =2212a(28) =2221a(29) =2222