cp's OEIS Frontend

A219041 Numbers k such that 3^k - 20 is prime.

%I A219041 #21 Nov 09 2023 20:13:39
%S A219041 3,4,5,6,10,11,19,20,23,25,26,71,80,91,101,139,150,179,200,246,599,
%T A219041 626,1126,2215,4189,7795,30626,66941,87630,104388
%N A219041 Numbers k such that 3^k - 20 is prime.
%C A219041 a(31) > 2*10^5. - _Robert Price_, Nov 14 2013
%e A219041 3^3 - 20 = 7 (prime), so 3 is in the sequence.
%t A219041 Do[If[PrimeQ[3^n - 20], Print[n]], {n, 3, 10000}]
%o A219041 (PARI) is(n)=isprime(3^n-20) \\ _Charles R Greathouse IV_, Feb 17 2017
%Y A219041 Cf. Sequences of numbers k such that 3^k + m is prime:
%Y A219041   (m = 2) A051783, (m = -2) A014224, (m = 4) A058958, (m = -4) A058959,
%Y A219041   (m = 8) A217136, (m = -8) A217135, (m = 10) A217137, (m = -10) A217347,
%Y A219041   (m = 14) A219035, (m = -14) A219038, (m = 16) A205647, (m = -16) A219039,
%Y A219041   (m = 20) A219040, (m = -20) A219041, (m = 22) A219042, (m = -22) A219043,
%Y A219041   (m = 26) A219044, (m = -26) A219045, (m = 28) A219046, (m = -28) A219047,
%Y A219041   (m = 32) A219048, (m = -32) A219049, (m = 34) A219050, (m = -34) A219051. Note that if m is a multiple of 3, 3^k + m is also a multiple of 3 (for k greater than 0), and as such isn't prime.
%K A219041 nonn,more
%O A219041 1,1
%A A219041 _Nicolas M. Perrault_, Nov 10 2012
%E A219041 a(27)-a(30) from _Robert Price_, Nov 14 2013