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%I #26 Feb 25 2020 18:26:07
%S 1,11,13,17,35,77,155,325,4097,5471,7151,7561,9211,13535,14627,18361,
%T 21815,41945,68255,105947,119915,358777,748745,843367,1171193,2730917
%N Numbers k such that 251*2^k+1 is prime.
%H Ray Ballinger, <a href="http://www.prothsearch.com/index.html">Proth Search Page</a>
%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>
%t Select[Range[1000], PrimeQ[251*2^# + 1] & ] (* _Robert Price_, Dec 20 2018 *)
%o (PARI) is(n)=ispseudoprime(251*2^n+1) \\ _Charles R Greathouse IV_, Jun 13 2017
%K nonn,hard,more
%O 1,2
%A _N. J. A. Sloane_.
%E a(21)-a(25) from the Ray Ballinger and Wilfrid Keller link by _Robert Price_, Dec 20 2018
%E a(26) from _Jeppe Stig Nielsen_, Feb 25 2020
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