A050566 - OEIS

%I #21 Jan 17 2019 10:08:17

%S 3,5,11,17,21,27,81,101,107,327,383,387,941,1665,3515,4007,4727,7137,

%T 16257,16833,21243,24239,35435,59925,91673,153455,165341,183167,

%U 190349,240743,253385,308325,741917

%N Numbers n such that 81*2^n-1 is prime.

%H Ray Ballinger and Wilfrid Keller, <a href="http://www.prothsearch.com/riesel1.html">List of primes k.2^n + 1 for k < 300</a>

%H Wilfrid Keller, <a href="http://www.prothsearch.com/riesel2.html">List of primes k.2^n - 1 for k < 300</a>

%H <a href="/index/Pri#riesel">Index entries for sequences of n such that k*2^n-1 (or k*2^n+1) is prime</a>

%H Kosmaj, <a href="http://www.15k.org/riesellist.html">Riesel list k<300</a>.

%t b=81; i=0; Table[While[i++; cp=b*2^i-1; !PrimeQ[cp]]; i, {j, 1, 17}]

%o (PARI) is(n)=ispseudoprime(81*2^n-1) \\ _Charles R Greathouse IV_, Feb 20 2017

%K hard,nonn

%O 1,1

%A _N. J. A. Sloane_, Dec 29 1999

%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

%E _Lei Zhou_, Nov 08 2013, added a Mathematica program for small elements.