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 A044971 #10 Jul 26 2023 03:20:51 %S A044971 3,11,15,19,21,35,47,51,59,61,65,69,73,75,107,143,155,159,179,185,187, %T A044971 197,209,213,221,223,227,231,235,237,323,431,467,479,483,539,557,563, %U A044971 565,593,629,641,645,665,671,673,683,695,699 %N A044971 Numbers whose base-3 representation contains exactly one 0 and one 1. %t A044971 Select[Range[1000],DigitCount[#,3,0]==DigitCount[#,3,1]==1&] (* _Harvey P. Dale_, Aug 06 2017 *) %Y A044971 Cf. A007089. %K A044971 nonn,base %O A044971 1,1 %A A044971 _Clark Kimberling_