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A245015
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Numbers n such that sigma(n) is a factorial.
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4
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1, 5, 14, 15, 23, 54, 56, 87, 95, 264, 270, 280, 354, 376, 406, 418, 435, 459, 478, 537, 623, 649, 667, 719, 1560, 1740, 1824, 1836, 1992, 2148, 2214, 2262, 2296, 2392, 2470, 2492, 2514, 2528, 2596, 2668, 2876, 3058, 3154, 3471, 3567, 3777, 3835, 3895, 4147, 4195, 4199
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OFFSET
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1,2
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COMMENTS
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LINKS
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EXAMPLE
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sigma(5) is 6 which is 3!, so 5 is n the sequence.
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MATHEMATICA
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Select[Range[4000], MemberQ[Range[8]!, DivisorSigma[1, #]] &] (* Alonso del Arte, Sep 24 2014 *)
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PROG
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(PARI) isok(n) = {if (n==1, return (1)); s = sigma(n); f = 1; for (k=2, s, f *= k; if (f == s, return (1)); if (f > s, return (0)); ); } \\ Michel Marcus, Mar 09 2015
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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