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A139204
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Numbers k such that (k!-9)/9 is prime.
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3
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6, 15, 17, 18, 21, 27, 29, 30, 37, 47, 50, 64, 125, 251, 602, 611, 1184, 1468, 5570, 10679, 15798, 21237
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OFFSET
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1,1
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COMMENTS
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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
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LINKS
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MATHEMATICA
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a = {}; Do[If[PrimeQ[(n! - 9)/9], Print[a]; AppendTo[a, n]], {n, 1, 300}]; a
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PROG
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(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
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CROSSREFS
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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).
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KEYWORD
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hard,more,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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