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Numbers n such that 2*23^n + 1 is prime.
4

%I #12 Feb 20 2017 14:52:51

%S 0,1,5,21,261,47589,93337

%N Numbers n such that 2*23^n + 1 is prime.

%C Primes found and proved by PrimeForm. No more terms up to 20000.

%C a(6) and a(7) proved prime by the primality proving program LLR. - _Robert Price_, Jan 06 2016

%C a(8) > 2*10^5. - _Robert Price_, Jan 06 2016

%t Join[{0}, Select[Range[1, 261, 2], PrimeQ[2*23^# + 1] &]] (* _Arkadiusz Wesolowski_, Nov 06 2012 *)

%o (PARI) is(n)=ispseudoprime(2*23^n+1) \\ _Charles R Greathouse IV_, Feb 20 2017

%Y Cf. A141774, A141797, A141802, A190942, A068231.

%K hard,nonn

%O 1,3

%A _Rick L. Shepherd_, Jul 05 2008

%E a(6)-a(7) from _Robert Price_, Jan 06 2016