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%I #45 Jun 06 2021 17:19:02
%S 0,1,3,17,143,261,551,2285,18731,18995,19751,62067,98051,169727,442281
%N Numbers k such that 5^k + 2 is a prime.
%C Terms <= 551 correspond to certified primes.
%C a(15) > 2*10^5. - _Robert Price_, Jan 16 2015
%H Henri & Renaud Lifchitz, <a href="http://www.primenumbers.net/prptop/searchform.php?form=5%5En%2B2&action=Search">PRP Records</a>.
%e a(3)=3 is a term because 5^3 + 2 = 127 is a prime.
%e 5^17 + 2 = 762939453127 is prime, hence 17 is a term.
%t Do[If[PrimeQ[5^n + 2], Print[n]], {n, 1, 10000}] (* _Ryan Propper_, Jun 17 2005 *)
%o (PARI) for(n=0, 10^5, if(ispseudoprime(5^n+2), print1(n, ", "))) \\ _Felix Fröhlich_, Jun 04 2014
%Y Cf. A051783 (3^n + 2 is prime).
%K hard,nonn
%O 1,3
%A _Donald S. McDonald_, Oct 13 2003
%E a(7)-a(8) from _Ryan Propper_, Jun 17 2005
%E a(9)-a(12) found by Mike Oakes in 2003. - _Alexander Adamchuk_, Mar 02 2008
%E Edited by _Ray Chandler_, Jul 27 2011
%E a(13) from _Ray Chandler_, Jul 28 2011
%E a(14) from _Robert Price_, Jan 16 2015
%E a(15) from _Paul Bourdelais_, Jan 28 2021
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