A202426 - OEIS

%I #28 Aug 14 2024 01:51:52

%S 2,3,4,10,11,13,17,22,26,36,79,196,1097,6926

%N Numbers k such that  k!*k!! - 1 is prime.

%C No additional terms up to k=1000. - _Harvey P. Dale_, Oct 29 2016

%e For k = 4, 4!*4!! - 1 = 24*8 - 1 = 191 is prime.

%t lst={}; Do[If[PrimeQ[n!*n!!-1], AppendTo[lst, n]], {n, 500}]; lst

%t Select[Range[200],PrimeQ[#!#!!-1]&] (* _Harvey P. Dale_, Oct 29 2016 *)

%o (Magma) a:=func< n | Factorial(n)*(&*[n..2 by -2])-1 >; [ n: n in [0..660] | IsPrime(a(n)) ];  // _Bruno Berselli_, Dec 19 2011

%Y Cf. A000142, A006882, A202424 (associated primes).

%K nonn,more

%O 1,1

%A _Michel Lagneau_, Dec 19 2011

%E a(13) from _Michael S. Branicky_, Jun 17 2023

%E a(14) from _Michael S. Branicky_, Aug 13 2024