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 A342079 #10 Feb 28 2021 10:11:03
%S A342079 1,2,21,3,4,41,5,6,61,7,8,81,9,11,12,22,23,13,14,42,24,43,15,16,62,25,
%T A342079 17,18,82,26,63,19,31,32,27,33,34,44,45,35,36,64,46,65,37,38,83,39,51,
%U A342079 52,28,84,47,53,54,48,85,55,56,66,67,57,58,86,68,87,59,71,72,29,73,74,49,75,76,69,77,78,88,89
%N A342079 Even digits only come in successive pairs (separated or not by a comma).
%C A342079 The sequence starts with a(1) = 1 and is always extended with the smallest positive integer not yet present that does not lead to a contradiction.
%C A342079 No term can end with an odd number of successive 0.
%e A342079 a(2) = 2 as the smallest positive integer not yet present that does not lead to a contradiction is 2;
%e A342079 a(3) = 21 (and not 20, as no term can end with an odd number of successive 0), because 21 is the smallest positive integer not yet present that completes a pair of identical even digits (2-2) and that does not lead to a contradiction;
%e A342079 a(4) = 3 as the smallest positive integer not yet present that does not lead to a contradiction is 3;
%e A342079 a(5) = 4 as the smallest positive integer not yet present that does not lead to a contradiction is 4;
%e A342079 a(6) = 41 (and not 40, as no term can end with an odd number of successive 0), because 41 is the smallest positive integer not yet present that completes a pair of identical even digits (4-4) and that does not lead to a contradiction; etc.
%o A342079 (Python) # see A342076 for aupton, pairsup
%o A342079 mustpair = {0, 2, 4, 6, 8}
%o A342079 print(aupton(67)) # _Michael S. Branicky_, Feb 28 2021
%Y A342079 Cf. A342076, A342077 and A342078 (variations on the same idea).
%K A342079 base,nonn
%O A342079 1,2
%A A342079 _Eric Angelini_, Feb 28 2021