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 A031011 #6 Sep 10 2024 13:17:55
%S A031011 3,13,27,36,38,40,41,42,44,46,48,55,69,83,100,121,142,155,158,161,163,
%T A031011 164,167,170,173,184,205,226,247,268,289,302,305,308,310,311,314,317,
%U A031011 320,331,352,373,387,390,393,394,396,399,402
%N A031011 Position of n-th 3 in A031007.
%t A031011 (Position[Flatten[Table[Reverse[IntegerDigits[n,7]],{n,0,200}]],3]//Flatten)-1 (* _Harvey P. Dale_, Sep 10 2024 *)
%K A031011 nonn
%O A031011 1,1
%A A031011 _Clark Kimberling_