OFFSET
1,1
EXAMPLE
k = 5 is here because 2*5! - 1 = 239 is prime.
MATHEMATICA
Select[Range[8000], PrimeQ[2 #! - 1] &] (* Vincenzo Librandi, Feb 20 2015 *)
PROG
(Magma) [n: n in [1..600] | IsPrime(2*Factorial(n)-1)]; // Vincenzo Librandi, Feb 20 2015
(PARI) is(k) = ispseudoprime(2*k!-1); \\ Jinyuan Wang, Feb 04 2020
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Phillip L. Poplin (plpoplin(AT)bellsouth.net), Oct 30 2002
EXTENSIONS
a(24)-a(29) from Serge Batalov, Feb 18 2015
STATUS
approved