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 A100028 #11 Jan 31 2023 12:31:03
%S A100028 3,5,7,9,23,29,33,185,267,307,757,897,1571,2977,3831,4595,6573,9511,
%T A100028 11651,15641,68885,69883
%N A100028 Values of n for which the decimal number 10...030...01 is an n-digit prime.
%F A100028 a(n) = 2*(A171376(n+1))+1. - _Chai Wah Wu_, Aug 20 2015
%e A100028 The corresponding primes are 131, 10301, 1003001, 100030001, 10000000000300000000001, etc.
%t A100028 IntegerLength/@Select[Table[FromDigits[Join[PadRight[{1},n,0],{3},PadLeft[ {1},n,0]]],{n,35000}],PrimeQ] (* _Harvey P. Dale_, Dec 20 2019 *)
%Y A100028 Cf. A100026, A100027, A100456, A100457, A100458, A100459.
%K A100028 nonn,base,more
%O A100028 1,1
%A A100028 Harvey Dubner (harvey(AT)dubner.com), Nov 20 2004