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 A185239 #15 Aug 04 2025 11:48:37 %S A185239 53,421,647,1601,35527 %N A185239 Numbers k such that (11^k + 10^k)/21 is prime. %C A185239 All terms are prime. %C A185239 a(6) > 10^5. %t A185239 Select[ Prime[ Range[1, 100000] ], PrimeQ[ (11^# + 10^#)/21 ]& ] %o A185239 (PARI) is(n)=ispseudoprime((11^n+10^n)/21) \\ _Charles R Greathouse IV_, May 22 2017 %Y A185239 Cf. A057178, A128341, A181141, A187819, A217095, A213216. %K A185239 nonn,more %O A185239 1,1 %A A185239 _Robert Price_, Apr 05 2013