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 A043506 #27 Apr 24 2021 21:16:55
%S A043506 44,144,244,344,404,414,424,434,440,441,442,443,445,446,447,448,449,
%T A043506 454,464,474,484,494,544,644,744,844,944,1044,1144,1244,1344,1404,
%U A043506 1414,1424,1434,1440,1441,1442,1443,1445,1446,1447,1448
%N A043506 Numbers having two 4's in base 10.
%H A043506 Daniel Starodubtsev, <a href="/A043506/b043506.txt">Table of n, a(n) for n = 1..10000</a>
%t A043506 Select[Range[100000], DigitCount[#, 10, 4] == 2 &] (* _Vincenzo Librandi_, Nov 20 2015 *)
%o A043506 (PARI) c(k, d, b) = {my(c=0, f); while (k>b-1, f=k-b*(k\b); if (f==d, c++); k\=b); if (k==d, c++); return(c)}
%o A043506 for(n=0, 2000, if(c(n, 4, 10)==2, print1(n, ", "))) \\ _Altug Alkan_, Nov 20 2015
%Y A043506 Cf. A043498.
%Y A043506 Subsequence of A011534.
%K A043506 nonn,base
%O A043506 1,1
%A A043506 _Clark Kimberling_