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 A291991 #9 Nov 11 2019 13:48:00
%S A291991 73,530713,6481,47763361,42521761,150094634909578633,
%T A291991 13490012358249728401,282429005041,
%U A291991 19966781110160346782368664772328944885905284750420567849,1076050302914923449767311155851656076154481
%N A291991 Primes of the form Phi(k, -9), where Phi is the cyclotomic polynomial.
%H A291991 Robert Price, <a href="/A291991/b291991.txt">Table of n, a(n) for n = 1..13</a>
%H A291991 Yves Gallot, <a href="http://yves.gallot.pagesperso-orange.fr/papers/cyclotomic.html">Cyclotomic polynomials and prime numbers</a>
%H A291991 <a href="/index/Cy#CyclotomicPolynomialsValuesAtX">Index entries for cyclotomic polynomials,values at X</a>
%t A291991 Select[Table[Cyclotomic[k,-9], {k, 0, 100}], PrimeQ[#] &]
%Y A291991 Cf. A138921.
%K A291991 nonn
%O A291991 1,1
%A A291991 _Robert Price_, Sep 07 2017