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 A031956 #13 Aug 10 2021 14:19:47 %S A031956 18,19,24,27,28,30,33,35,36,39,44,45,49,50,52,54,56,57,66,67,70,71,72, %T A031956 73,74,76,77,79,82,83,88,91,92,94,96,97,98,100,103,104,107,109,110, %U A031956 111,112,113,115,116,118,121,122,123,124,126,129 %N A031956 Numbers with exactly three distinct base-4 digits. %Y A031956 Cf. A007090. %K A031956 nonn,base %O A031956 1,1 %A A031956 _Clark Kimberling_