A090669 - OEIS

%I #31 Jul 07 2024 13:20:34

%S 1,2,4,7,8,12,15,28,31,84,98,128,238,302,859,1508,1586,2091,2796,2888,

%T 3924,4815,5636,6596,7090,20176,22176,56386,84050,115515,245608,

%U 259710,274120

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

%e 7^12 - 2 = 13841287199 a prime number.

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

%o (PARI) is(n)=isprime(7^n - 2) \\ _Charles R Greathouse IV_, Apr 28 2015

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

%K nonn,more

%O 1,2

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

%E Corrected and extended by _Robert G. Wilson v_ and _Ray Chandler_, Dec 22 2003

%E Three more terms from _Ryan Propper_, Dec 07 2008

%E a(28)-a(30) from _Robert Price_, Jan 24 2014

%E a(31)-a(32) from _Paul Bourdelais_, Jan 29 2021

%E a(33) from _Paul Bourdelais_, Aug 02 2023