A082672 Numbers n such that (n! + 2)/2 is a prime.

2, 4, 5, 7, 8, 13, 16, 30, 43, 49, 91, 119, 213, 1380, 1637, 2258, 4647, 9701, 12258

Offset: 1

Author

Cino Hilliard, May 18 2003

Keywords

• easy
• nonn

Crossrefs

Cf. A089130.

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).

Programs

Magma

[ n: n in [1..300] | IsPrime((Factorial(n)+2) div 2) ];

Mathematica

Select[Range[10^2], PrimeQ[(#!+2)/2] &] (* Vladimir Joseph Stephan Orlovsky, Apr 29 2008 *)

PARI

\\ x such that (x!+2)/2 is prime
xfactpk(n,k=2) = { for(x=2,n, y = (x!+k)/k; if(isprime(y),print1(x, ", ")) ) }

Extensions

More terms from Don Reble, Dec 08 2003

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