A090669 Numbers k such that 7^k - 2 is a prime.

1, 2, 4, 7, 8, 12, 15, 28, 31, 84, 98, 128, 238, 302, 859, 1508, 1586, 2091, 2796, 2888, 3924, 4815, 5636, 6596, 7090, 20176, 22176, 56386, 84050, 115515, 245608, 259710, 274120

Offset

1,2

Links

Table of n, a(n) for n=1..33.

Example

7^12 - 2 = 13841287199 a prime number.

Mathematica

Do[ If[ PrimeQ[7^n - 2], Print[n]], {n, 1, 2000}] (* Robert G. Wilson v, Dec 22 2003 *)

Prog

(PARI) is(n)=isprime(7^n - 2) \\ Charles R Greathouse IV, Apr 28 2015

Crossrefs

Cf. A236371, A217131, A191469, A090669, A096305, A217130, A217132, A152213.

Sequence in context: A233271 A088413 A372098 * A231428 A263660 A215822

Adjacent sequences: A090666 A090667 A090668 * A090670 A090671 A090672

Keyword

nonn,more

Author

Herman H. Rosenfeld (herm3(AT)pacbell.net), Dec 16 2003

Extensions

Corrected and extended by Robert G. Wilson v and Ray Chandler, Dec 22 2003

Three more terms from Ryan Propper, Dec 07 2008

a(28)-a(30) from Robert Price, Jan 24 2014

a(31)-a(32) from Paul Bourdelais, Jan 29 2021

a(33) from Paul Bourdelais, Aug 02 2023

Status

approved