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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 (list; graph; refs; listen; history; text; internal format)
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

See A046069

LINKS

Table of n, a(n) for n=1..97.

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: A055095 A337618 A048685 * A128979 A332509 A336157

Adjacent sequences:  A101047 A101048 A101049 * A101051 A101052 A101053

KEYWORD

nonn

AUTHOR

Pierre CAMI, Jan 21 2005

EXTENSIONS

Corrected and extended by T. D. Noe, Aug 04 2005

STATUS

approved

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Last modified July 24 20:26 EDT 2021. Contains 346273 sequences. (Running on oeis4.)