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A052333
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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.
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12
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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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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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 (warut822(AT)yahoo.com). This was proved to be a prime by Paul Jobling (Paul.Jobling(AT)WhiteCross.com) using PrimeForm and by Ignacio Larrosa Canestro (ignacio.larrosa(AT)eresmas.net) using Titanix (http://www.znz.freesurf.fr/pages/titanix.html). [Oct 30 2000]
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LINKS
| Ray Ballinger and Wilfrid Keller, The Riesel Problem: Definition and Status
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EXAMPLE
| a(4)=19 because 4 -> 9 (composite) -> 19 (prime).
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MATHEMATICA
| Table[NestWhile[2#+1&, 2n+1, !PrimeQ[#]&], {n, 60}] (* From Harvey P. Dale, May 08 2011 *)
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PROG
| (PARI) a(n)=while(!isprime(n=2*n+1), ); n \\ Charles R Greathouse IV, May 08, 2011
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CROSSREFS
| Cf. A050412 (values of n), A051914, A052334, A052339, A052340, A040081.
Sequence in context: A064080 A184875 A112986 * A074106 A002261 A154524
Adjacent sequences: A052330 A052331 A052332 * A052334 A052335 A052336
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KEYWORD
| nonn,nice
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AUTHOR
| Christian G. Bower (bowerc(AT)usa.net), Dec 19 1999
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