This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 (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 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. 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). Sequence in context: A009579 A114812 A158338 * A122661 A133481 A274549 Adjacent sequences:  A139201 A139202 A139203 * A139205 A139206 A139207 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified June 20 19:36 EDT 2019. Contains 324234 sequences. (Running on oeis4.)