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 A217647 Numbers n such that 2*n!!! + 1 is prime. 4
 0, 1, 2, 3, 6, 12, 15, 20, 21, 44, 54, 75, 102, 116, 182, 288, 326, 344, 357, 458, 1296, 2394, 2913, 4749 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS n!!! is a triple factorial, see the definition in A007661. LINKS EXAMPLE 6 is in the sequence because 2*6 !!! + 1 = 2*18 + 1 = 37 is prime. MAPLE 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: MATHEMATICA 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 PROG (PARI) is(n)=ispseudoprime(2*prod(i=0, (n-2)\3, n-3*i)+1) \\ Charles R Greathouse IV, Oct 09 2012 (PFGW) ABC2 2*\$a!3+1 a: from 1 to 5000 Charles R Greathouse IV, Oct 09 2012 CROSSREFS Cf. A007661. Sequence in context: A252792 A284638 A111271 * A070926 A226023 A124485 Adjacent sequences:  A217644 A217645 A217646 * A217648 A217649 A217650 KEYWORD nonn,hard AUTHOR Michel Lagneau, Oct 09 2012 EXTENSIONS a(22)-a(23) from Charles R Greathouse IV, Oct 09 2012 STATUS approved

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Last modified September 18 09:40 EDT 2019. Contains 327168 sequences. (Running on oeis4.)