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A101050
Least k such that prime(n)*2^k-1 is prime, or -1 if no such k exists.
2
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
OFFSET
1,3
COMMENTS
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
REFERENCES
MATHEMATICA
Table[p=Prime[n]; k=0; While[ !PrimeQ[ -1+p*2^k], k++ ]; k, {n, 119}] (* T. D. Noe, Aug 04 2005 )
CROSSREFS
Cf. A046069 (least k such that (2n-1)*2^k-1 is prime).
Sequence in context: A337618 A048685 A364663 * A128979 A332509 A336157
KEYWORD
nonn
AUTHOR
Pierre CAMI, Jan 21 2005
EXTENSIONS
Corrected and extended by T. D. Noe, Aug 04 2005
STATUS
approved