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 A098645 #16 Dec 13 2019 13:38:53 %S A098645 1,3,10,20,22,31,32,33,34,35,41,51,52,53,54,55,111,112,200,210,220, %T A098645 222,231,1111,2000,2002,2003,2004,2005,2006,2007,2008,2009,2020,2022, %U A098645 2023,2024,2031,10000,20000,20002,20003,20004,20005,20006,20007,20008,20009 %N A098645 List the positions of all digits '1' in the sequence. This is the lexicographically earliest increasing sequence with this property. %C A098645 Start with a(1) = 1. For n > 1, choose a(n) to be the smallest number > a(n-1) consistent with the condition that "the a(n)-th digit is a 1 and no 1's occur in other positions" is true for all n. %C A098645 If we change "> a(n-1)" to "not already used", we get 1, 3, 10, 6, 11, 7, 21, 13, 15, 17, 19, 101, ... - _David Wasserman_, Feb 26 2008 %C A098645 That sequence has by now been entered as A210415. - _M. F. Hasler_, Oct 08 2013 %H A098645 <a href="/index/Se#self-referencing_sequences">Index to the OEIS: Entries related to self-referencing sequences</a>. %e A098645 The first digit of the sequence is a "1", the 3rd digit also, then the 10th, the 11th, etc. %Y A098645 Cf. A098670. See A114134 for another version. See also A210414-A210423. %K A098645 base,easy,nonn %O A098645 1,2 %A A098645 _Eric Angelini_, Oct 27 2004 %E A098645 Entry revised by Eric Angelini and _N. J. A. Sloane_, Feb 03 2006. %E A098645 More terms from _David Wasserman_, Feb 26 2008