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 A046866 Numbers k such that 6*7^k - 1 is prime. 13
 0, 1, 2, 7, 18, 55, 69, 87, 119, 141, 189, 249, 354, 1586, 2135, 2865, 2930, 4214, 7167, 67485, 74402, 79326 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS a(23) > 2*10^5. - Robert Price, Nov 13 2015 REFERENCES R. K. Guy, Unsolved Problems in Number Theory, Section A3. LINKS H. C. Williams, The primality of certain integers of the form 2Ar^n - 1, Acta Arith. 39 (1981), 7-17. MATHEMATICA Do[ If[ PrimeQ[6*7^n - 1], Print[n]], {n, 0, 5650}] PROG (PARI) for(n=0, 2000, if(isprime(6*7^n-1), print1(n, ", "))) CROSSREFS Cf. A003307, A002235, A046865, A079906, A001771, A005541, A056725, A046867, A079907. Sequence in context: A099626 A261331 A046672 * A291255 A294004 A214836 Adjacent sequences:  A046863 A046864 A046865 * A046867 A046868 A046869 KEYWORD nonn,hard,more AUTHOR EXTENSIONS One more term from Jason Earls, Jul 21 2001 More terms from Robert G. Wilson v, Jan 17 2003 One more term from Ryan Propper, Jun 05 2006 a(20)-a(22) from Donovan Johnson, Nov 26 2008 First term 0 inserted by Georg Fischer, Aug 01 2019 STATUS approved

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Last modified July 28 15:51 EDT 2021. Contains 346335 sequences. (Running on oeis4.)