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 A051915 #41 Jun 09 2025 19:44:17 %S A051915 0,1,2,3,5,12,18,35,51,53,78,209,396,4166,9091,9587,13357,15917,17652, %T A051915 46127,66480 %N A051915 Numbers k such that 2*k! + 1 is prime. %C A051915 Used PrimeForm to prove primality for n = 4166 (classical N-1 test). - _David Radcliffe_, May 28 2007 %C A051915 a(22) > 80000. - _Serge Batalov_, Jun 09 2025 %e A051915 k = 5 is here because 2*5! + 1 = 241 is prime. %t A051915 Select[Range[0, 400], PrimeQ[2*#! + 1] &] (* _Vladimir Joseph Stephan Orlovsky_, Feb 13 2012 *) %o A051915 (Magma) [n: n in [0..1000] | IsPrime(2*Factorial(n) +1)]; // _Vincenzo Librandi_, Feb 21 2015 %o A051915 (PARI) is(k) = ispseudoprime(2*k!+1); \\ _Jinyuan Wang_, Feb 05 2020 %Y A051915 Cf. A002981, A076679, A076680, A076681, A076682, A076683, A178488, A180626, A126896. %K A051915 nonn,more %O A051915 1,3 %A A051915 _Labos Elemer_, Dec 18 1999 %E A051915 4166 from _David Radcliffe_, May 28 2007 %E A051915 More terms from _Serge Batalov_, Feb 18 2015 %E A051915 a(21) from _Serge Batalov_, Jun 08 2025