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 A261578 Numbers m such that (4^m + 17) / 3 is prime. 1

%I

%S 1,2,5,8,11,23,26,59,83,89,116,1103,1568,5768,13376,17810,18614,66209,

%T 167933,188318

%N Numbers m such that (4^m + 17) / 3 is prime.

%C After 1, m is of the form 3*k+2. In fact, for m = 3*k or 3*k+1, 4^n+17 is divisible by 9 and 7, respectively. [_Bruno Berselli_, Aug 26 2015]

%C a(21) > 300000. - _Robert Price_, Apr 04 2017

%e 2 is in the sequence because (4^2+17)/3 = 11 is prime.

%e 5 is in the sequence because (4^5+17)/3 = 347 is prime.

%t Select[Range[0, 5000], PrimeQ[(4^# + 17)/3] &]

%o (MAGMA) [n: n in [0..1000] | IsPrime((4^n+17) div 3)];

%o (PARI) for(n=1, 1e3, if(isprime((4^n+17)/3), print1(n", "))) \\ _Altug Alkan_, Sep 14 2015

%Y Cf. similar sequences listed in A261539.

%K nonn,more

%O 1,2

%A _Vincenzo Librandi_, Aug 26 2015

%E a(14)-a(15) from _Vincenzo Librandi_, Sep 14 2015

%E a(16)-a(20) from _Robert Price_, Feb 01 2017

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Last modified December 10 14:27 EST 2019. Contains 329896 sequences. (Running on oeis4.)