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 A121242 #10 Jun 13 2018 23:09:48
%S A121242 1,1,1,1,1,1,1,1,2,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,4,4,4,4,
%T A121242 4,4,4,4,4,4,4,4,4,4,4,4,5,7,9,11,12,13,14,15,16,17,18,19,20,21,22,23,
%U A121242 23,23,23,23,23,23,23,23,23,23,23,23,23,23,23,23,23,23,23,24,24,24,24,24
%N A121242 Number of 2's in first n primes.
%C A121242 In the first 10^m (m=3,4,5) primes there are 339, 4070, 55213 2's, among all 3803, 48982, 610484 digits, which gives the frequencies {0.08914, 0.08309, 0.09044}.
%H A121242 Robert Israel, <a href="/A121242/b121242.txt">Table of n, a(n) for n = 1..10000</a>
%p A121242 ListTools:-PartialSums([seq(numboccur(2,convert(ithprime(i),base,10)),i=1..100)]); # _Robert Israel_, Jun 13 2018
%t A121242 Accumulate[DigitCount[Prime[Range[90]],10,2]] (* _Harvey P. Dale_, Nov 09 2013 *)
%Y A121242 Cf. A068670 Number of digits in the first n primes.
%Y A121242 Partial sums of A085976.
%K A121242 base,nonn
%O A121242 1,9
%A A121242 _Zak Seidov_, Aug 22 2006