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 A178437 #16 Jan 17 2019 13:44:08
%S A178437 2,3,17,19,31,37,43,91,339,367,407,1135,1175,8653,11987,15793
%N A178437 Numbers n such that 10^n - 81 is prime.
%C A178437 The next term, if one exists, is >100000. - _Robert Price_, Apr 25 2011
%H A178437 Makoto Kamada, <a href="https://stdkmd.net/nrr/aaaba.htm">Near-repdigit numbers of the form AA...AABA</a>.
%H A178437 Makoto Kamada, <a href="https://stdkmd.net/nrr/9/99919.htm#prime">Prime numbers of the form 99...9919</a>.
%H A178437 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e A178437 10^3 - 81 = 919 which is a prime number.
%t A178437 nMax=1000; Do[If[PrimeQ[10^n-81], Print[n]], {n, nMax}]
%o A178437 (PARI) is(n)=ispseudoprime(10^n-81) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A178437 Cf. A108330, A108328.
%K A178437 more,nonn
%O A178437 1,1
%A A178437 _Robert Price_, Dec 21 2010