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 A210418 #20 Apr 18 2021 02:15:03 %S A210418 2,4,5,44,7,40,11,41,14,17,42,21,43,24,27,45,31,46,34,37,47,54,444, %T A210418 4444,48,52,404,57,400,62,401,67,402,72,403,77,405,82,406,87,407,92, %U A210418 408,97,409,103,410,109,411,114,118,412,124,413,123,133,414,135,140 %N A210418 List the positions of all digits 4 in the concatenation of all terms, not necessarily in order. This is the lexicographically earliest such sequence. %C A210418 See A210415 for comments, links, and code. %H A210418 Danny Rorabaugh, <a href="/A210418/b210418.txt">Table of n, a(n) for n = 1..1000</a> %e A210418 The sequence cannot start with 1 because the first digit is not 4. Let us start with 2: the second digit is 4. So in the second position we have 4. The third digit cannot be 3 because the third digit is not 4. So let it be 5. In the next position we now have 44. And so on. %Y A210418 Cf. A210414, A210415, A210416, A210417, A210419, A210420, A210421, A210422, A210423. %K A210418 nonn,base %O A210418 1,1 %A A210418 _Paolo P. Lava_, Mar 26 2012 %E A210418 Name edited and a(22)-a(59) corrected by _Danny Rorabaugh_, Nov 30 2015