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A082387 Numbers k such that (5^k + 2^k)/7 is prime. 25

%I

%S 3,17,19,47,101,1709,2539,5591,6037,8011,19373,26489,27427

%N Numbers k such that (5^k + 2^k)/7 is prime.

%C Next term > 35000. - _Giovanni Resta_, Apr 06 2006

%C For (2^k + 5^k)/7 to be a prime, k must be a prime too. - _Alexander Adamchuk_, Feb 06 2007

%C No other terms less than 100000. - _Robert Price_, Apr 17 2012

%H Primeform, <a href="http://groups.yahoo.com/group/primeform/">User group for PFGW and PrimeForm programs</a>

%e a(1)=3 because (5^3 + 2^3)/7 = (125 + 8)/7 = 19 is prime.

%t Do[f=(2^n+5^n)/7; If[PrimeQ[f], Print[{n, f}]], {n, 1, 35000}] (* _Alexander Adamchuk_, Feb 06 2007 *)

%t Select[Range[0, 10000], PrimeQ[(5^# + 2^#)/7] &] (* _G. C. Greubel_, Nov 02 2018 *)

%o (PARI) is(n)=ispseudoprime((5^n + 2^n)/7) \\ _Charles R Greathouse IV_, Apr 28 2015

%o (MAGMA) [n : n in [1..10000] | IsPrime((5^n+2^n) div 7)]; // _G. C. Greubel_, Nov 02 2018

%Y Cf. A057469 ((2^k + 3^k)/5 is prime).

%K more,nonn

%O 1,1

%A _Hugo Pfoertner_, May 22 2003

%E More terms from _Hugo Pfoertner_, Jun 23 2003

%E More terms from _Giovanni Resta_, Apr 06 2006

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Last modified June 16 10:03 EDT 2019. Contains 324152 sequences. (Running on oeis4.)