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 A231591 #14 Jun 16 2019 13:31:28
%S A231591 1,3,32,391,3906,39572,400626,4047829,40794211,410514052,4126066282,
%T A231591 41436122092,415853103290,4171375888398
%N A231591 Total number of 2's digits in primes less than 10^n.
%e A231591 a(2)=3, since there are 3 2's in primes less than 100. Namely: 2, 23, 29.
%t A231591 Table[Count[IntegerDigits[Prime[Range[PrimePi[10^n - 1]]]], 2, 2], {n, 7}] (* _Robert Price_, Jun 16 2019 *)
%Y A231591 Cf. A231411, A231590-A231598, A119290, A119291-A119300.
%K A231591 nonn,base,more
%O A231591 1,2
%A A231591 _Robert Price_, Nov 11 2013
%E A231591 a(14) from _Giovanni Resta_, Jul 20 2015