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 A123902 #17 Dec 10 2015 03:51:12
%S A123902 0,1,12,100,112,121,122,123,1012,1023,1102,1122,1123,1200,1201,1203,
%T A123902 1213,1222,1223,1231,1232,1233,1234,10000,10012,10023,10102,10123,
%U A123902 10201,10202,10203,10213,10223,10231,10232,10234,11023,11102,11200,11203,11211,11213
%N A123902 A123896 sorted and duplicates removed.
%D A123902 D. E. Knuth, The Art of Computer Programming, vol. 4A, Combinatorial Algorithms, Section 7.2.1.5, p. 432, Problems 4 and 5.
%H A123902 R. J. Mathar, <a href="/A123902/b123902.txt">Table of n, a(n) for n = 0..4460</a>
%t A123902 f[n_] := Block[{d = Prepend[IntegerDigits@ n, 0], a, b, w}, b = DeleteDuplicates@ d; a = Range[0, Length@ b]; w = FromDigits@ Flatten[Part[a, FirstPosition[b, #]] & /@ d]; w]; Take[Union@ Table[f[n^2], {n, 0, 320}], 42] (* _Michael De Vlieger_, Dec 09 2015, Version 10 *)
%K A123902 nonn,base
%O A123902 0,3
%A A123902 _N. J. A. Sloane_, Nov 20 2006
%E A123902 More terms from _Franklin T. Adams-Watters_, Nov 20 2006