A082671 - OEIS

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

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OFFSET

1,1

LINKS

EXAMPLE

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

MATHEMATICA

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

PROG

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

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

CROSSREFS

KEYWORD

hard,more,nonn

AUTHOR

EXTENSIONS

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

Edited by T. D. Noe, Oct 30 2008

STATUS

approved