This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A113698 #8 Aug 03 2014 14:01:22
%S A113698 1,2,12,3,13,23,123,4,14,24,34,124,134,234,1234,5,15,25,35,45,125,135,
%T A113698 145,235,245,345,1235,1245,1345,2345,12345,6,16,26,36,46,56,126,136,
%U A113698 146,156,236,246,256,346,356,456,1236,1246,1256,1346,1356,1456,2346,2356
%N A113698 Combinatorial sequence. Begin with 1 then 2 then 12 then 3 then all concatenations of all sizes of 1,2 and 3, then 4, then all concatenations of all sizes of 1,2,3,4 not included earlier etc.
%C A113698 The index of n is 2^n for n<10. After 9 if n ( like 13, 23) has appeared earlier it will not appear but it will be used in the concatenation at its turn as mentioned above. needs better description.
%C A113698 The sequence contains groups of integers generated from seeds s=1,2,3,4,... A group is the sorted list of numbers defined by the seed and all concatenations of integers of previous groups with the seed, discarding any duplicates. - _R. J. Mathar_, Aug 31 2007
%H A113698 T. D. Noe, <a href="/A113698/b113698.txt">Table of n, a(n) for n = 1..511</a>
%e A113698 The group 4, 14, 24, 34, 124, 134, 234, 1234 is generated from the seed s=4 itself and attaching s=4 to the previous elements 1, 2, 12, 3, 13, 23, 123, that is 14, 24, 124, 34, 134, 234, 1234, then sorting within the group (moving 34 between 24 and 124).
%t A113698 Flatten[Table[FromDigits /@ Complement[Subsets[Range[n]], Subsets[Range[n - 1]]], {n, 5}]] (* _T. D. Noe_, Feb 22 2012 *)
%Y A113698 Cf. A113699.
%K A113698 base,nonn
%O A113698 1,2
%A A113698 _Amarnath Murthy_, Nov 11 2005
%E A113698 More terms from _R. J. Mathar_, Aug 31 2007