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A217647
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Numbers k such that 2*k!!! + 1 is prime.
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4
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0, 1, 2, 3, 6, 12, 15, 20, 21, 44, 54, 75, 102, 116, 182, 288, 326, 344, 357, 458, 1296, 2394, 2913, 4749, 6834, 10425, 17448
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OFFSET
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1,3
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COMMENTS
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k!!! is a triple factorial, see the definition in A007661.
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LINKS
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EXAMPLE
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6 is in the sequence because 2*6!!! + 1 = 2*18 + 1 = 37 is prime.
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MAPLE
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A:= n -> mul(k, k = select(k -> k mod 3 = n mod 3, [$1 .. n])): for p from 0 to 200 do:if type(2*A(p)+1, prime)=true then printf(`%d, `, p):else fi:od:
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MATHEMATICA
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lst={}; multiFactorial[n_, k_] := If[n < 1, 1, If[n < k + 1, n, n*multiFactorial[n - k, k]]]; Do[If[PrimeQ[2*multiFactorial[n, 3] + 1], AppendTo[lst, n]], {n, 0, 1000}]; lst
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PROG
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(PFGW)
ABC2 2*$a!3+1
a: from 1 to 5000
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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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STATUS
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approved
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