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A100614
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Numbers n such that (!n)/2 is prime, where !n = Sum_{k=0..n-1} k!.
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3
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3, 4, 5, 8, 9, 10, 11, 30, 76, 163, 271, 273, 354, 721, 1796, 3733, 4769, 9316, 12221, 41532
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OFFSET
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1,1
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COMMENTS
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No other terms below 50000. - Serge Batalov, Jul 23 2017
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REFERENCES
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R. K. Guy, Unsolved Problems In Number Theory, B44.
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LINKS
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Table of n, a(n) for n=1..20.
Yves Gallot, Is the number of primes (of half the left handed factorials) finite?.
Eric Weisstein's World of Mathematics, Left Factorial
Eric Weisstein's World of Mathematics, Integer Sequence Primes
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FORMULA
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When A014288(n-1) is prime.
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MATHEMATICA
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s = 1; Do[s = s + n!; If[ PrimeQ[s/2], Print[n + 1]], {n, 10^3}] (* Robert G. Wilson v, Dec 02 2004 *)
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CROSSREFS
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Cf. A014288, Left factorials: A003422.
See A124375 for another version.
Sequence in context: A112594 A188003 A120519 * A274231 A173999 A127427
Adjacent sequences: A100611 A100612 A100613 * A100615 A100616 A100617
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KEYWORD
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nonn,more
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AUTHOR
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R. K. Guy, Dec 02 2004
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EXTENSIONS
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a(14) from Robert G. Wilson v, Dec 02 2004
a(15)=1796 from Ray Chandler, Dec 02 2004
a(17) from T. D. Noe, Dec 04 2004
Corrected by adding a(16)=3733 from Eric W. Weisstein, Oct 29 2005
a(18)=9316 from Eric W. Weisstein, Dec 27 2005
a(19)=12221 from Eric W. Weisstein, Oct 19 2006
a(20)=41532 from Serge Batalov, Jul 22 2017
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STATUS
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approved
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