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%I #23 Mar 07 2022 13:35:46
%S 3,5,7,19,11,13,31,17,19,43,23,103,223,29,31,67,71,37,79,41,43,367,47,
%T 199,103,53,223,463,59,61,127,131,67,139,71,73,151,311,79,163,83,5503,
%U 738197503,89,367,751,191,97,199,101,103,211,107,109,223,113,463
%N Riesel problem: start with n; repeatedly double and add 1 until reach a prime. Sequence gives prime reached, or 0 if no prime is ever reached.
%C Smallest prime of form (n+1)*2^k-1 for k >= 1 (or 0 if no such prime exists).
%C a(509202)=0 (i.e. never reaches a prime) - Chris Nash (chris_nash(AT)hotmail.com). (Of course this is related to the entry 509203 of A076337.)
%C a(73) is a 771-digit prime reached after 2552 iterations - _Warut Roonguthai_. This was proved to be a prime by Paul Jobling (Paul.Jobling(AT)WhiteCross.com) using PrimeForm and by _Ignacio Larrosa Cañestro_ using Titanix (http://www.znz.freesurf.fr/pages/titanix.html). [Oct 30 2000]
%H Ray Ballinger and Wilfrid Keller, <a href="http://www.prothsearch.com/rieselprob.html">The Riesel Problem: Definition and Status</a>
%e a(4)=19 because 4 -> 9 (composite) -> 19 (prime).
%t Table[NestWhile[2#+1&,2n+1,!PrimeQ[#]&],{n,60}] (* _Harvey P. Dale_, May 08 2011 *)
%o (PARI) a(n)=while(!isprime(n=2*n+1),);n \\ oo loop when a(n) = 0. - _Charles R Greathouse IV_, May 08 2011
%Y Cf. A050412 (values of n), A051914, A052334, A052339, A052340, A040081.
%K nonn,nice
%O 1,1
%A _Christian G. Bower_, Dec 19 1999
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