A046865 - OEIS

%I #46 Apr 05 2019 05:43:20

%S 0,1,3,9,13,15,25,39,69,165,171,209,339,2033,6583,15393,282989,498483,

%T 504221,754611,864751

%N Numbers k such that 4*5^k - 1 is prime.

%C a(22) > 1000000. - _Karsten Bonath_, Apr 04 2019

%D R. K. Guy, Unsolved Problems in Number Theory, Section A3.

%H Steven Harvey, <a href="http://harvey563.tripod.com/wills.txt">Williams Primes</a>

%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 Print[0]; Do[ If[ PrimeQ[4*5^n - 1], Print[n]], {n, 1, 8100, 2}]

%o (PARI) is(n)=isprime(4*5^n-1) \\ _Charles R Greathouse IV_, Feb 07 2017

%Y Cf. A003307, A002235, A079906, A046866, A001771, A005541, A056725, A046867, A079907.

%K nonn,hard,nice,more

%O 1,3

%A _N. J. A. Sloane_

%E Two more terms from _Robert G. Wilson v_, Jan 16 2003 and Jan 26 2003

%E a(16) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 05 2008

%E a(1)=0 prepended by _Robert Price_, Feb 27 2015

%E a(17) from _Karsten Bonath_, Dec 07 2018

%E a(18)-a(19) from _Karsten Bonath_, Jan 17 2019

%E a(20)-a(21) from _Karsten Bonath_, Apr 04 2019