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%I #12 Nov 01 2018 21:49:30
%S 5,23,73,101,401,419,457,811,1163,1511,8011
%N Numbers k such that (2^k + 7^k)/9 is prime.
%C All terms are primes. Corresponding primes of the form (2^k + 7^k)/9 are {1871, 3040971926676589439, 5469081705798319217773539465593130845206220817280793349743311, ...}.
%C a(12) > 10^5. - _Robert Price_, Aug 28 2012
%t Do[p=Prime[n];f=(2^p+7^p)/9; If[PrimeQ[f], Print[{p, f}]], {n, 1, 1000}]
%o (PARI) is(n)=ispseudoprime((2^n+7^n)/9) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A000978 = numbers n such that (2^n + 1)/3 is prime. Cf. A057469 = numbers n such that (2^n + 3^n)/5 is prime. Cf. A082387 = numbers n such that (2^n + 5^n)/7 is prime.
%K hard,more,nonn
%O 1,1
%A _Alexander Adamchuk_, Feb 06 2007
%E More terms from _Ryan Propper_, Mar 23 2007
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