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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

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

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

N. J. A. Sloane

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 January 22 10:25 EST 2020. Contains 331144 sequences. (Running on oeis4.)