A050856 - OEIS

%I #22 Jan 17 2019 10:08:17

%S 4,8,16,28,40,52,80,148,328,8464,11704,52372,60172,675232,692692,

%T 921484

%N Numbers k such that 209*2^k-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 Kosmaj, <a href="http://www.15k.org/riesellist.html">Riesel list 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>

%t Select[Range[1000], PrimeQ[209*2^# - 1] & ] (* _Robert Price_, Dec 29 2018 *)

%o (PARI) is(n)=ispseudoprime(209*2^n-1) \\ _Charles R Greathouse IV_, Jun 13 2017

%K hard,nonn,more

%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 a(16) from the Wilfrid Keller link by _Robert Price_, Dec 29 2018