A082671 Numbers n such that (n!-2)/2 is a prime.

3, 4, 5, 6, 9, 31, 41, 373, 589, 812, 989, 1115, 1488, 1864, 1918, 4412, 4686, 5821, 13830

Offset: 1

Author

Cino Hilliard, May 18 2003

Keywords

• hard
• more
• nonn

Examples

			(4!-2)/2 = 11 is a prime.
		

Crossrefs

Cf. n!/m-1 is a prime: A002982, A082671, A139056, A139199, A139200, A139201, A139202, A139203, A139204, A139205; n!/m+1 is a prime: A002981, A082672, A089085, A139061, A139058, A139063, A139065, A151913, A137390, A139071

Programs

Magma

[n: n in [1..600]| IsPrime((Factorial(n)-2) div 2)]; // Vincenzo Librandi, Feb 18 2015

Mathematica

Select[Range[0, 14000], PrimeQ[(#! - 2) / 2] &] (* Vincenzo Librandi, Feb 18 2015 *)

PARI

xfactpk(n,k=2) = { for(x=2,n, y = (x!-k)/k; if(isprime(y),print1(x", ")) ) }

Extensions

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008

Edited by T. D. Noe, Oct 30 2008