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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. 22
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 and Eric Chen, Table of n, a(n) for n = 1..2292 (first 1000 terms from T. D. Noe)

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

(PARI) a(n)=for(k=0, 2^16, if(ispseudoprime(n*2^k-1), return(k))) \\ Eric Chen, Jun 01 2015

CROSSREFS

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

Cf. A046069 (for odd n)

Cf. A010051, A000079.

Sequence in context: A155103 A048105 A176202 * A239393 A256637 A113063

Adjacent sequences:  A040078 A040079 A040080 * A040082 A040083 A040084

KEYWORD

nonn

AUTHOR

David W. Wilson

STATUS

approved

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Last modified August 29 12:40 EDT 2015. Contains 261193 sequences.