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A128344 Numbers k such that (7^k - 5^k)/2 is prime. 20


%S 3,5,7,113,397,577,7573,14561,58543

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

%C All terms are primes.

%C No other terms less than 10^5. - _Robert Price_, May 28 2012

%t k=7; Do[p=Prime[n]; f=(k^p-5^p)/(k-5); If[ PrimeQ[f], Print[p] ], {n,1,100}]

%o (PARI) is(n)=isprime((7^n-5^n)/2) \\ _Charles R Greathouse IV_, Feb 17 2017

%Y Cf. A062572, A128345, A128346, A128347, A128348, A128349, A128350, A128351, A128352, A128353, A128354. Cf. A004061, A082182, A121877, A059802. Cf. A057171, A082387, A122853, A128335, A128336, A128337, A128338, A128339, A128340, A128341, A128342.

%K hard,more,nonn

%O 1,1

%A _Alexander Adamchuk_, Feb 27 2007

%E a(7)-a(9) from _Robert Price_, May 28 2012

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Last modified February 17 00:25 EST 2020. Contains 331976 sequences. (Running on oeis4.)