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A040081 Riesel problem: a(n) = smallest m >= 0 such that n*2^m-1 is prime, or -1 if no such prime exists. 20
2, 1, 0, 0, 2, 0, 1, 0, 1, 1, 2, 0, 3, 0, 1, 1, 2, 0, 1, 0, 1, 1, 4, 0, 3, 2, 1, 3, 4, 0, 1, 0, 2, 1, 2, 1, 1, 0, 3, 1, 2, 0, 7, 0, 1, 3, 4, 0, 1, 2, 1, 1, 2, 0, 1, 2, 1, 3, 12, 0, 3, 0, 2, 1, 4, 1, 5, 0, 1, 1, 2, 0, 7, 0, 1, 1, 2, 2, 1, 0, 3, 1, 2, 0, 5, 6, 1, 23, 4, 0, 1, 2, 3, 3, 2, 1, 1, 0, 1, 1, 10, 0, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

T. D. Noe, Table of n, a(n) for n=1..1000

MATHEMATICA

Table[m = 0; While[! PrimeQ[n*2^m - 1], m++]; m, {n, 100}] (* Arkadiusz Wesolowski, Sep 04 2011 *)

PROG

(Haskell)

a040081 = length . takeWhile ((== 0) . a010051) .

                       iterate  ((+ 1) . (* 2)) . (subtract 1)

-- Reinhard Zumkeller, Mar 05 2012

CROSSREFS

Cf. A038699 (primes obtained), A050412, A052333.

Cf. A046069 (for odd n)

Cf. A010051, A000079.

Sequence in context: A155103 A048105 A176202 * A239393 A113063 A123477

Adjacent sequences:  A040078 A040079 A040080 * A040082 A040083 A040084

KEYWORD

nonn

AUTHOR

David W. Wilson

STATUS

approved

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Last modified August 29 08:27 EDT 2014. Contains 246187 sequences.