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A194636
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Least k >= 0 such that (2*n-1)*2^k - 1 or (2*n-1)*2^k + 1 is prime, or -1 if no such value exists.
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13
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0, 0, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 4, 1, 1, 1, 1, 1, 5, 3, 1, 1, 2, 1, 2, 2, 1, 2, 1, 1, 1, 4, 1, 1, 1, 2, 1, 1, 1, 3, 3, 1, 3, 6, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 3, 1, 2, 1, 1, 2, 5, 1, 3, 4, 1, 1, 1, 1, 2, 2, 1, 4, 1, 2
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OFFSET
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1,7
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COMMENTS
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Bisection of A194591: a(n) = A194591(2*n-1).
A194637 gives the record values.
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LINKS
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Arkadiusz Wesolowski, Table of n, a(n) for n = 1..1000
Eric Weisstein's World of Mathematics, Brier Number
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EXAMPLE
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For n=4, 7*2^0-1 and 7*2^0+1 are composite, but 7*2^1-1=13 is prime, so a(4)=1.
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MATHEMATICA
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Table[n = 2*n - 1; k = 0; While[! PrimeQ[n*2^k - 1] && ! PrimeQ[n*2^k + 1], k++]; k, {n, 100}] (* Arkadiusz Wesolowski, Sep 04 2011 *)
p[n_]:=Module[{c=2n-1, k=0}, While[!Or@@PrimeQ[c*2^k+{1, -1}], k++]; k]; Array[ p, 90] (* Harvey P. Dale, Mar 08 2013 *)
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CROSSREFS
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Cf. A194591, A194600, A194603, A194606, A194607, A194608, A194635, A194637, A194638, A194639.
Cf. A040081, A040076, A076335, A180247.
Sequence in context: A144477 A106345 A319395 * A286299 A081729 A219157
Adjacent sequences: A194633 A194634 A194635 * A194637 A194638 A194639
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KEYWORD
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sign
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AUTHOR
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Arkadiusz Wesolowski, Aug 31 2011
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STATUS
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approved
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