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A215779
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Numbers k such that 2*k!! - 1 is prime.
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3
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2, 3, 5, 9, 11, 13, 25, 35, 46, 56, 68, 69, 71, 84, 92, 103, 110, 121, 193, 259, 308, 368, 406, 426, 620, 627, 1206, 1631, 1845, 2108, 2210, 2786
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OFFSET
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1,1
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COMMENTS
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The corresponding primes are in A215780.
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LINKS
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EXAMPLE
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5 is in the sequence because 2*5!! - 1 = 2*15 - 1 = 29 is a prime.
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MATHEMATICA
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lst={}; Do[If[PrimeQ[2*n!!-1], AppendTo[lst, n]], {n, 0, 1500}]; lst
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CROSSREFS
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KEYWORD
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nonn,hard,more
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AUTHOR
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EXTENSIONS
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Offset changed to 1 by and a(27)-a(32) from Jinyuan Wang, May 15 2021
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STATUS
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approved
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