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 A031949 #15 Aug 10 2021 18:46:40 %S A031949 4,6,7,8,9,11,12,13,14,16,17,20,22,23,25,26,29,31,32,34,37,38,40,41, %T A031949 43,46,47,48,51,53,55,58,59,60,61,62,64,65,68,69,80,81,84,86,87,89,90, %U A031949 93,95,101,102,105,106,117,119,125,127,128,130 %N A031949 Numbers with exactly two distinct base-4 digits. %t A031949 db4Q[n_]:=Count[DigitCount[n,4],_?(#>0&)]==2; Select[Range[200],db4Q] (* _Harvey P. Dale_, Jan 13 2019 *) %Y A031949 Cf. A007090. %K A031949 nonn,base %O A031949 1,1 %A A031949 _Clark Kimberling_