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 A291990 #12 Nov 11 2019 13:47:53
%S A291990 101,9091,909091,9901,909090909090909091,99990001,
%T A291990 909090909090909090909090909091,999999000001,1111111111111111111,
%U A291990 11111111111111111111111,9999999900000001,9090909090909090909090909090909090909090909090909091
%N A291990 Primes of the form Phi(k, -10), where Phi is the cyclotomic polynomial.
%H A291990 Robert Price, <a href="/A291990/b291990.txt">Table of n, a(n) for n = 1..18</a>
%H A291990 Yves Gallot, <a href="http://yves.gallot.pagesperso-orange.fr/papers/cyclotomic.html">Cyclotomic polynomials and prime numbers</a>
%H A291990 <a href="/index/Cy#CyclotomicPolynomialsValuesAtX">Index entries for cyclotomic polynomials,values at X</a>
%t A291990 Select[Table[Cyclotomic[k,-10], {k, 3, 100}], PrimeQ[#] &]
%Y A291990 Cf. A138920.
%K A291990 nonn
%O A291990 1,1
%A A291990 _Robert Price_, Sep 07 2017