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A040081
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Riesel problem: a(n) = smallest m >= 0 such that n*2^m-1 is prime, or -1 if no such prime exists.
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24
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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;
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refs;
listen;
history;
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internal format)
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OFFSET
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1,1
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LINKS
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T. D. Noe and Eric Chen, Table of n, a(n) for n = 1..2292 (first 1000 terms from T. D. Noe)
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MATHEMATICA
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Table[m = 0; While[! PrimeQ[n*2^m - 1], m++]; m, {n, 100}] (* Arkadiusz Wesolowski, Sep 04 2011 *)
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PROG
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(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
(Python)
from sympy import isprime
def a(n):
m = 0
while not isprime(n*2**m - 1): m += 1
return m
print([a(n) for n in range(1, 88)]) # Michael S. Branicky, Feb 01 2021
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CROSSREFS
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Cf. A038699 (primes obtained), A050412, A052333.
Cf. A046069 (for odd n)
Cf. A010051, A000079.
Sequence in context: A048105 A335021 A176202 * A239393 A256637 A113063
Adjacent sequences: A040078 A040079 A040080 * A040082 A040083 A040084
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KEYWORD
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nonn
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AUTHOR
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David W. Wilson
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STATUS
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approved
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