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A238904
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Smallest k such that 2^k + (2n+1) and (2n+1)*2^k + 1 are both prime, k <= n, or -1 if no such k exists.
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2
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0, 1, 1, 2, 1, 1, 2, 1, -1, 6, 1, -1, 2, 2, 1, -1, 6, 1, 2, 1, 1, 2, 9, -1, 2, 1, -1, 4, 2, -1, 12, 4, 1, 2, 1, 3, 6, 3, -1, 2, 1, -1, 4, 6, 9, 8, 2, 1, 2, 1, 3, -1, 1, -1, 6, 1, -1, 12, 6, 3, 12, 8, 1, 2, 3, 3, 4, 1, -1, -1, 3, -1, -1, 60, 3, 4, 2, 1, 12
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OFFSET
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0,4
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LINKS
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EXAMPLE
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a(0) = 0 because 2^0 + (2*0+1) = 2 and (2*0+1)*2^0 + 1 = 2 are both prime.
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MATHEMATICA
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a[n_] := Catch@Block[{k=0}, While[k <= n, If[PrimeQ[2^k + 2*n + 1] && PrimeQ[(2*n + 1)*2^k + 1], Throw@k]; k++]; -1]; a/@ Range[0, 80] (* Giovanni Resta, Mar 15 2014 *)
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CROSSREFS
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KEYWORD
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sign
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AUTHOR
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STATUS
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approved
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