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 A103317 Primes p such that p! - 1 is also prime. 5
 3, 7, 379, 6917, 208003 (list; graph; refs; listen; history; text; internal format)
 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 Table of n, a(n) for n=1..5. Eric Weisstein's World of Mathematics, Factorial Prime Index entries for sequences related to factorial numbers. 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 Cf. A002982, A093804. Sequence in context: A348376 A089689 A248701 * A104051 A128004 A281889 Adjacent sequences: A103314 A103315 A103316 * A103318 A103319 A103320 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

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Last modified March 2 00:03 EST 2024. Contains 370447 sequences. (Running on oeis4.)