A139200 - OEIS

%I #17 Sep 08 2022 08:45:33

%S 5,11,12,16,36,41,42,47,127,136,356,829,1863,2065,2702,4509,7498

%N Numbers k such that (k!-5)/5 is prime.

%C a(16) > 3000. - _Ray G. Opao_, Oct 05 2008

%C a(18) > 25000. - _Robert Price_, Nov 20 2016

%t a = {}; Do[If[PrimeQ[(n! - 5)/5], Print[a]; AppendTo[a, n]], {n, 1, 300}]; a (* _Artur Jasinski_ *)

%o (Magma) [n: n in [5..500] | IsPrime((Factorial(n)-5) div 5)]; // _Vincenzo Librandi_, Nov 21 2016

%Y Cf. A139189, A139190, A139191, A139192, A139193, A139194, A139195, A139196, A139197, A139198.

%Y Cf. n!/m-1 is a prime: A002982, A082671, A139056, A139199-A139205; n!/m+1 is a prime: A002981, A082672, A089085, A139061, A139058, A139063, A139065, A151913, A137390, A139071 (1<=m<=10).

%K hard,more,nonn

%O 1,1

%A _Artur Jasinski_, Apr 11 2008

%E a(13)-a(15) from _Ray G. Opao_, Oct 05 2008

%E a(16) from _Serge Batalov_, Feb 18 2015

%E a(17) from _Robert Price_, Nov 20 2016