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 A210421 #21 Apr 17 2021 22:19:52 %S A210421 2,7,4,70,8,77,11,71,15,72,19,73,23,74,27,75,26,33,76,37,78,36,43,79, %T A210421 47,700,46,54,701,59,702,64,703,69,777,7777,77770,83,704,87,90,705,95, %U A210421 706,101,707,103,110,708,116,709,122,710,127,131,711,137,712,136 %N A210421 List the positions of all digits 7 in the concatenation of all terms, not necessarily in order. This is the lexicographically earliest such sequence. %C A210421 See A210415 for comments, links, and code. %H A210421 Danny Rorabaugh, <a href="/A210421/b210421.txt">Table of n, a(n) for n = 1..1000</a> %e A210421 The sequence cannot start with 1 because the first digit is not 7. Let us start with 2: the second digit is equal to 7. The third digit cannot be 3 because the third digit is not 7. So let it be 4. In the next position we now have 70 because it is the minimum number greater than 7 and starting with the digit 7. And so on. %Y A210421 Cf. A210414, A210415, A210416, A210417, A210418, A210419, A210420, A210422, A210423. %K A210421 nonn,base %O A210421 1,1 %A A210421 _Paolo P. Lava_, Mar 26 2012 %E A210421 Name edited and a(5)-a(31) corrected by _Danny Rorabaugh_, Nov 30 2015