A139204 - OEIS

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A139204

Numbers k such that (k!-9)/9 is prime.

6, 15, 17, 18, 21, 27, 29, 30, 37, 47, 50, 64, 125, 251, 602, 611, 1184, 1468, 5570, 10679, 15798, 21237

(list; graph; refs; listen; history; text; internal format)

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

LINKS

Table of n, a(n) for n=1..22.

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

Cf. A139189, A139190, A139191, A139192, A139193, A139194, A139195, A139196, A139197, A139198.