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 A102503 #9 Jul 28 2020 17:40:01
%S A102503 910109,913139,916169,917179,920209,923239,928289,931319,935359,
%T A102503 937379,946469,953539,956569,961619,965659,968689,974749,982829,
%U A102503 994949,995959,998989,9101010109,9101310139,9101810189,9104210429,9104810489
%N A102503 Primes of the concatenated form 9nn9.
%H A102503 Harvey P. Dale, <a href="/A102503/b102503.txt">Table of n, a(n) for n = 1..2500</a>
%e A102503 910109 is prime and of the form 9nn9 for n=10.
%e A102503 9101310139 is prime and of the form 9nn9 for n=1013.
%t A102503 Select[Table[FromDigits[Join[{9},IntegerDigits[n],IntegerDigits[n],{9}]],{n,1100}],PrimeQ] (* _Harvey P. Dale_, Jul 28 2020 *)
%Y A102503 Cf. A102484 for sequence of all numbers of form 9nn9. A102502 for the n values corresponding to the primes in this sequence.
%K A102503 easy,nonn,base
%O A102503 1,1
%A A102503 Mark Hudson (mrmarkhudson(AT)hotmail.com), Jan 13 2005