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 A178439 #14 Jan 17 2019 13:44:08
%S A178439 4,5,7,27,91,248,347,501,535,767,2002,2581,2677,8158,15977,20666,
%T A178439 23854,72730,86816
%N A178439 Numbers n such that 10^n - 99 is prime.
%C A178439 The next term, if it exists, is > 100000.
%C A178439 See Kamada link - primecount.txt for terms, primesize.txt for discovery details including probable or proved primes - search on "99901".
%H A178439 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/">List of near-repdigit-related prime numbers</a>.
%H A178439 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e A178439 10^4 - 99 = 9901 which is a prime number.
%t A178439 nMax=1000; Do[If[PrimeQ[10^n-99], Print[n]], {n, nMax}]
%o A178439 (PARI) is(n)=ispseudoprime(10^n-99) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A178439 Cf. A108330, A108328.
%K A178439 more,nonn
%O A178439 1,1
%A A178439 _Robert Price_, Dec 21 2010
%E A178439 a(18-19)=72730,86816 from _Robert Price_, May 29 2011