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 A100456 #18 Jan 31 2023 12:29:29
%S A100456 3,5,9,11,17,55,331,461,475,739,971,1257,1751,1929,2839,8167
%N A100456 Values of n for which the decimal number 10...050...01 is an n-digit prime.
%C A100456 a(17) > 53125. - _Robert Price_, Apr 14 2013
%C A100456 Three is a term because 151 is a prime, thus illustrating that zeros are optional in the resulting primes. - _Harvey P. Dale_, Oct 08 2019
%t A100456 IntegerLength/@Select[Table[FromDigits[Join[PadRight[{1},n,0],{5},PadLeft[ {1},n,0]]],{n,5000}],PrimeQ] (* _Harvey P. Dale_, Oct 08 2019 *)
%Y A100456 Cf. A100028.
%K A100456 nonn,base,more
%O A100456 1,1
%A A100456 Harvey Dubner (harvey(AT)dubner.com), Nov 23 2004
%E A100456 Corrected by _T. D. Noe_, Oct 25 2006