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 A043520 #12 Apr 24 2021 21:18:14 %S A043520 7777,17777,27777,37777,47777,57777,67777,70777,71777,72777,73777, %T A043520 74777,75777,76777,77077,77177,77277,77377,77477,77577,77677,77707, %U A043520 77717,77727,77737,77747,77757,77767,77770,77771,77772 %N A043520 Numbers having four 7's in base 10. %H A043520 Daniel Starodubtsev, <a href="/A043520/b043520.txt">Table of n, a(n) for n = 1..10000</a> %Y A043520 Subsequence of A011537. %K A043520 nonn,base %O A043520 1,1 %A A043520 _Clark Kimberling_