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 A228240 #23 Feb 16 2025 08:33:20
%S A228240 1,2,40,242,842,1541,75067
%N A228240 Integer lengths of log(10)-primes.
%C A228240 Next term > 76902. - _Eric W. Weisstein_, Oct 12 2015
%H A228240 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ConstantPrimes.html">Constant Primes</a>
%H A228240 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/IntegerSequencePrimes.html">Integer Sequence Primes</a>
%H A228240 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/NaturalLogarithmof10Digits.html">Natural Logarithm of 10 Digits</a>
%F A228240 a(n) = IntegerLength(A228241(n)).
%e A228240 log(10) = 2.302585093..., so
%e A228240 a(1) = 1 (1-digit number 2 is prime),
%e A228240 a(2) = 2 (2-digit number 23 is a prime),
%e A228240 a(3) = 40 (40-digit number 2302585092994045684017991454684364207601 is prime).
%t A228240 Module[{nn=2000,lg10},lg10=RealDigits[Log[10],10,nn][[1]];IntegerLength/@ Select[Table[FromDigits[Take[lg10,n]],{n,nn}],PrimeQ]] (* _Harvey P. Dale_, Jul 16 2015 *)
%Y A228240 Cf. A228241 (log(10)-primes).
%Y A228240 Cf. A002392 (decimal digits of log(10)).
%K A228240 nonn,base,more
%O A228240 1,2
%A A228240 _Eric W. Weisstein_, Aug 17 2013
%E A228240 a(7) from _Eric W. Weisstein_, Oct 12 2015