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A139204
Numbers k such that (k!-9)/9 is prime.
3
6, 15, 17, 18, 21, 27, 29, 30, 37, 47, 50, 64, 125, 251, 602, 611, 1184, 1468, 5570, 10679, 15798, 21237
OFFSET
1,1
COMMENTS
a(20) > 10000. The PFGW program has been used to certify all the terms up to a(19), using a deterministic test which exploits the factorization of a(n) + 1. - Giovanni Resta, Mar 28 2014
a(23) > 25000. - Robert Price, Mar 29 2017
MATHEMATICA
a = {}; Do[If[PrimeQ[(n! - 9)/9], Print[a]; AppendTo[a, n]], {n, 1, 300}]; a
PROG
(PARI) for(n=1, 1000, if(floor(n!/9-1)==n!/9-1, if(ispseudoprime(n!/9-1), print(n)))) \\ Derek Orr, Mar 28 2014
CROSSREFS
KEYWORD
hard,more,nonn
AUTHOR
Artur Jasinski, Apr 11 2008
EXTENSIONS
a(14)-a(16) from Derek Orr, Mar 28 2014
a(17)-a(19) from Giovanni Resta, Mar 28 2014
a(20)-a(22) from Robert Price, Mar 29 2017
STATUS
approved