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 A309589 #9 Apr 10 2019 10:48:06
%S A309589 0,1,2,3,4,5,6,7,8,9,0,1,10,11,12,13,14,15,16,17,18,19,2,20,21,22,23,
%T A309589 24,25,26,27,28,29,3,30,31,32,33,34,35,36,37,38,39,4,40,41,42,43,44,
%U A309589 45,46,47,48,49,5,50,51,52,53,54,55,56,57,58,59,6,60,61
%N A309589 Number subsets {0, ..., 10^k - 1} written in base 10 and sorted lexicographically, for k = 1, 2, ...
%C A309589 The sequence is the flattened form of an irregular table T(k, i). The rows for k >= 1 contain a permutation of the numbers 0 <= i <= 10^k - 1 which is defined by the lexicographical order of the numbers i written in base 10.
%C A309589 This "useless" order appears, for example, in a directory listing of numbered filenames, or after an ASCII sort of signatures of linear recurrences. The Perl program in the link computes this sequence and variations with different ranges and bases.
%H A309589 Georg Fischer, <a href="/A309589/a309589.txt">Perl program</a> which generates this sequence and its inverse.
%e A309589 Table T(k, i) begins:
%e A309589 k\i 0 1 2 3 ...
%e A309589 -------------------------
%e A309589 1: 0 1 2 3 ... 9
%e A309589 2: 0 1 10 11 ... 19 2 20 21 ... 99
%e A309589 3: 0 1 10 100 ... 109 11 110 111 ... 999
%e A309589 4: ...
%o A309589 (Perl) # cf. link
%Y A309589 Cf. A119589 (like row k=2, but 1 <= i <= 100), A190016 (like row k=4, but 1 <= i <= 10000), A309590 (inverse)
%K A309589 nonn,base,easy,tabf
%O A309589 0,3
%A A309589 _Georg Fischer_, Mar 02 2019