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A101050
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Least k such that prime(n)*2^k-1 is prime, or -1 if no such k exists.
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2
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1, 0, 2, 1, 2, 3, 2, 1, 4, 4, 1, 1, 2, 7, 4, 2, 12, 3, 5, 2, 7, 1, 2, 4, 1, 10, 3, 10, 9, 8, 25, 2, 2, 1, 4, 5, 1, 3, 4, 2, 8, 3, 226, 3, 2, 1, 1, 3, 2, 1, 4, 4, 11, 6, 4, 2, 8, 1, 5, 2, 11, 2, 1, 26, 3, 6, 1, 1, 18, 3, 4, 4, 1, 7, 1, 2, 20, 5, 10, 3, 4, 7, 2, 3, 1, 6, 112, 9, 10, 7, 2, 12, 5, 46, 1, 2, 8
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OFFSET
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1,3
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COMMENTS
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Primes p such that p*2^k-1 is composite for all k are called Riesel numbers. The smallest known Riesel number is the prime 509203. Currently, 2293 is the smallest prime whose status is unknown. For a(120), which corresponds to the prime 659, Dave Linton found the least k is 800516. - T. D. Noe, Aug 04 2005
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REFERENCES
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See A046069
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LINKS
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Table of n, a(n) for n=1..97.
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MATHEMATICA
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Table[p=Prime[n]; k=0; While[ !PrimeQ[ -1+p*2^k], k++ ]; k, {n, 119}] (* T. D. Noe, Aug 04 2005 )
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CROSSREFS
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Cf. A046069 (least k such that (2n-1)*2^k-1 is prime).
Sequence in context: A355249 A337618 A048685 * A128979 A332509 A336157
Adjacent sequences: A101047 A101048 A101049 * A101051 A101052 A101053
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KEYWORD
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nonn
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AUTHOR
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Pierre CAMI, Jan 21 2005
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EXTENSIONS
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Corrected and extended by T. D. Noe, Aug 04 2005
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STATUS
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approved
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