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 A215779 Numbers n such that 2*n!! - 1 is prime. 3
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS The corresponding primes are in A215780. LINKS EXAMPLE 5 is in the sequence because 2*5!!+1 = 2*15 +1 = 31 is a prime. MATHEMATICA lst={}; Do[If[PrimeQ[2*n!!-1], AppendTo[lst, n]], {n, 0, 1500}]; lst CROSSREFS Cf. A006882, A215780. Sequence in context: A014657 A171056 A161514 * A140329 A163292 A137518 Adjacent sequences:  A215776 A215777 A215778 * A215780 A215781 A215782 KEYWORD nonn,hard AUTHOR Michel Lagneau, Aug 23 2012 STATUS approved

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Last modified December 9 00:32 EST 2019. Contains 329871 sequences. (Running on oeis4.)