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 A073780 #13 Feb 23 2015 09:20:49
%S A073780 0,1,1,1,1,3,1,1,1,1,3,3,3,3,1,1,1,1,3,1,1,1,1,1,3,1,3,1,3,3,3,3,1,3,
%T A073780 3,3,3,1,1,1,1,1,1,3,1,3,3,1,1,3,1,1,1,1,3,1,1,3,3,3,5,3,3,3,3,1,3,5,
%U A073780 3,3,3,3,5,5,3,3,3,3,3,3,3,3,1,3,3,3,3,3
%N A073780 Number of 1's in base 3 representation of n-th prime.
%H A073780 Harvey P. Dale, <a href="/A073780/b073780.txt">Table of n, a(n) for n = 1..1000</a>
%F A073780 a(n) = A062756(A000040(n)). - _Michel Marcus_, Oct 02 2013
%e A073780 a1(11)=3 as 11th prime, 31, is 1011 in base 3 representation.
%t A073780 a1[n_] := Length[Cases[IntegerDigits[Prime[n], 3], 1]]
%t A073780 Table[DigitCount[n,3,1],{n,Prime[Range[100]]}] (* _Harvey P. Dale_, Feb 23 2015 *)
%Y A073780 Cf. A073779, A073781.
%K A073780 base,easy,nonn
%O A073780 1,6
%A A073780 _Zak Seidov_ Aug 11 2002