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Numbers k such that 2*7^k - 3 is prime.
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%I #14 Jul 10 2024 18:39:14

%S 1,3,4,7,132,253,9040,21888

%N Numbers k such that 2*7^k - 3 is prime.

%C a(9) > 27376. - _Jinyuan Wang_, Dec 21 2019

%C a(9) > 60000. - _Michael S. Branicky_, Jul 10 2024

%t Do[ If[ PrimeQ[ 2*7^n - 3 ], Print[n] ], {n, 1, 6000} ]

%o (PARI) is(n)=ispseudoprime(2*7^n-3) \\ _Charles R Greathouse IV_, Jun 13 2017

%Y Cf. A059041 (3*7^k+2 is prime).

%K nonn,more

%O 1,2

%A _Robert G. Wilson v_, Feb 12 2001

%E a(7)-a(8) from _Jinyuan Wang_, Dec 21 2019