OFFSET
1,1
COMMENTS
The members are the primes in A002982 (n! - 1 is prime).
REFERENCES
R. K. Guy, Unsolved Problems in Number Theory, Section A2.
LINKS
Eric Weisstein's World of Mathematics, Factorial Prime
EXAMPLE
3 is prime and 3! - 1 = 5 is prime, so 3 is a member.
MATHEMATICA
Select[Prime[Range[100]], PrimeQ[#! - 1] &] (* Vladimir Joseph Stephan Orlovsky, Feb 01 2012 *)
PROG
(PARI) isok(n) = ispseudoprime(n) && ispseudoprime(n!-1); \\ Jinyuan Wang, Jan 20 2020
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Jonathan Sondow, Jan 31 2005
EXTENSIONS
a(5)=208003 (found on Jul 27 2016, by Sou Fukui), added by Jinyuan Wang, Jan 20 2020
STATUS
approved