A046866 Numbers k such that 6*7^k - 1 is prime.

0, 1, 2, 7, 18, 55, 69, 87, 119, 141, 189, 249, 354, 1586, 2135, 2865, 2930, 4214, 7167, 67485, 74402, 79326, 231349

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn
• hard
• more

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.

Crossrefs

Cf. A003307, A002235, A046865, A079906, A001771, A005541, A056725, A046867, A079907.

Programs

Mathematica

Do[ If[ PrimeQ[6*7^n - 1], Print[n]], {n, 0, 5650}]

PARI

for(n=0,2000, if(isprime(6*7^n-1),print1(n, ", ")))

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

a(23) from Riley Fisher, Dec 02 2024