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Numbers k such that (19^k - 4^k)/15 is prime.
0

%I #22 Mar 07 2021 02:12:59

%S 2,7,11,31,47,139,263,769,20551,21491,112247

%N Numbers k such that (19^k - 4^k)/15 is prime.

%C All terms are prime.

%C a(12) > 2*10^5.

%t Select[Prime[Range[1, 100000]], PrimeQ[(19^# - 4^#)/15]&]

%o (PARI) is(n)=ispseudoprime((19^n-4^n)/15) \\ _Charles R Greathouse IV_, Jun 13 2017

%Y Cf. A224691, A128354, A128032.

%K nonn,hard,more

%O 1,1

%A _Robert Price_, Aug 09 2013

%E a(11) from _Robert Price_, Mar 06 2021