%I A081242
%S A081242 1,2,11,21,12,22,111,211,121,221,112,212,122,222,1111,2111,1211,2211,
%T A081242 1121,2121,1221,2221,1112,2112,1212,2212,1122,2122,1222,2222,11111,
%U A081242 21111,12111,22111,11211,21211,12211,22211,11121,21121,12121,22121
%N A081242 Left-to-right binary enumeration.
%C A081242 In A007931, the arithmetic is done from right to left, yielding reversals 
               of the terms of A081242. In A007931, new wordlengths occur at 1,3,
               7,15,...; in A081242, they occur at 2,4,8,16,.... In A007931, indexing 
               starts at 1 and the sequence is numerical; in A081242, indexing starts 
               at 2, leaving room for the empty word at position 1 and the sequence 
               consists of all binary words.
%F A081242 Write n in base 2, remove initial 1, add 1 to remaining digits and reverse 
               their order. Or, start with empty word e, follow by 1 and then successively 
               add 1, always working from left to right.
%e A081242 Eleven in base 2 is 1011; remove initial 1 to have 011; add 1 to all 
               digits to have 122; reverse those to have a(11)=221. Or, start with 
               the empty word and add 1's successively, doing the arithmetic from 
               left to right: e -> 1 -> 2 -> 11 -> 21 -> 22 -> 111 -> 211 -> ...
%Y A081242 Cf. A059893, A007931.
%Y A081242 Sequence in context: A061907 A052216 A094629 this_sequence A071762 A113721 
               A127199
%Y A081242 Adjacent sequences: A081239 A081240 A081241 this_sequence A081243 A081244 
               A081245
%K A081242 base,nonn,word
%O A081242 2,2
%A A081242 Clark Kimberling (ck6(AT)evansville.edu), Mar 12 2003