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 A303784 #11 Dec 01 2024 17:46:57 %S A303784 1,10,2,12,3,14,4,16,5,18,6,20,7,22,8,24,9,26,100,11,102,13,104,15, %T A303784 106,17,108,19,110,21,112,23,114,25,116,27,118,28,120,29,122,30,124, %U A303784 31,126,32,128,33,130,34,132,35,134,36,136,37,138,38,140,39,142,40,144,41,146,42,148,43,150,44,152,45,154,46,156,47 %N A303784 Lexicographically earliest sequence of distinct terms such that what emerges from the mask is even (see the Comment section for the mask explanation). %C A303784 For any pair of contiguous terms, one of the terms uses fewer digits than the other. This term is called the mask. Put the mask on the other term, starting from the left. What is not covered by the mask forms an even number. %C A303784 The sequence starts with a(1) = 1 and is always extended with the smallest integer not yet present that doesn't lead to a contradiction. %C A303784 This sequence is a permutation of the integers > 0, as all integers will appear at some point, either as mask or masked. %H A303784 Jean-Marc Falcoz, <a href="/A303784/b303784.txt">Table of n, a(n) for n = 1..10001</a> %e A303784 In the pair (1,10), 1 is the mask; 0 emerges and is even; %e A303784 In the pair (10,2), 2 is the mask; 0 emerges and is even; %e A303784 In the pair (2,12), 2 is the mask; 2 emerges and is even; %e A303784 In the pair (12,3), 3 is the mask; 2 emerges and is even; %e A303784 ... %e A303784 In the pair (690,2018), 690 is the mask; 8 emerges and is even; %e A303784 etc. %Y A303784 Cf. A303782 (same idea with primes), A303783 (with squares), A303785 (with odd numbers), A303786 (rebuilds term by term the sequence itself). %K A303784 nonn,base %O A303784 1,2 %A A303784 _Eric Angelini_ and _Jean-Marc Falcoz_, Apr 30 2018