A052333 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.

3, 5, 7, 19, 11, 13, 31, 17, 19, 43, 23, 103, 223, 29, 31, 67, 71, 37, 79, 41, 43, 367, 47, 199, 103, 53, 223, 463, 59, 61, 127, 131, 67, 139, 71, 73, 151, 311, 79, 163, 83, 5503, 738197503, 89, 367, 751, 191, 97, 199, 101, 103, 211, 107, 109, 223, 113, 463

Offset

1,1

Comments

Smallest prime of form (n+1)*2^k-1 for k >= 1 (or 0 if no such prime exists).

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.)

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]

Links

Table of n, a(n) for n=1..57.

Ray Ballinger and Wilfrid Keller, The Riesel Problem: Definition and Status

Example

a(4)=19 because 4 -> 9 (composite) -> 19 (prime).

Mathematica

Table[NestWhile[2#+1&, 2n+1, !PrimeQ[#]&], {n, 60}] (* Harvey P. Dale, May 08 2011 *)

Prog

(PARI) a(n)=while(!isprime(n=2*n+1), ); n \\ oo loop when a(n) = 0. - Charles R Greathouse IV, May 08 2011

Crossrefs

Cf. A050412 (values of n), A051914, A052334, A052339, A052340, A040081.

Sequence in context: A064080 A184875 A112986 * A074106 A002261 A263257

Adjacent sequences: A052330 A052331 A052332 * A052334 A052335 A052336

Keyword

nonn,nice

Author

Christian G. Bower, Dec 19 1999

Status

approved