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%I #35 Nov 21 2023 15:38:49
%S 1,2,6,7,11,23,33,48,68,79,116,151,205,1016,1332,1448,3481,3566,3665,
%T 11233,13363,29166,44358,58530,191706,386450,605168,616879,1204077
%N Numbers k such that 5*6^k - 1 is prime.
%C a(29) > 618000. - _Karsten Bonath_, Nov 04 2019
%D R. K. Guy, Unsolved Problems in Theory of Numbers, Section A3.
%H H. C. Williams, <a href="http://matwbn.icm.edu.pl/ksiazki/aa/aa39/aa3912.pdf">The primality of certain integers of the form 2Ar^n - 1</a>, Acta Arith. 39 (1981), 7-17.
%t Do[ If[ PrimeQ[5*6^n - 1], Print[n]], {n, 1, 5000}]
%o (PARI) for(n=1,2000, if(isprime(5*6^n-1),print1(n, ", ")))
%Y Cf. A003307, A002235, A046865, A046866, A001771, A005541, A056725, A046867, A079907, A247260.
%K nonn,hard,more
%O 1,2
%A _Robert G. Wilson v_, Jan 16 2003
%E a(20)-a(24) from _Donovan Johnson_, Nov 26 2008
%E a(25) from _Robert Price_, Jan 23 2016
%E a(26) from _Karsten Bonath_, Jul 01 2019
%E a(27) from _Karsten Bonath_, Oct 29 2019
%E a(28) from _Karsten Bonath_, Nov 04 2019
%E a(29) from _Ryan Propper_, Nov 21 2023