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A327599
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Odd numbers k that have a divisor d such that sigma(d)*d is equal to k.
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3
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1, 117, 775, 2793, 9801, 16093, 30927, 88723, 90675, 137541, 292537, 326781, 488125, 732511, 796797, 954273, 1882881, 1926183, 2164575, 2896363, 3500157, 3618459, 4985713, 6725201, 7595775, 8042167, 10380591, 12326221, 12472075, 14076543, 16092297, 20456373, 23968425, 25774633
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OFFSET
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1,2
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COMMENTS
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We need d and sigma(d) odd which happens precisely when d is an odd square.
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LINKS
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EXAMPLE
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As 9 * sigma(9) = 9 * (1 + 3 + 9) = 9 * 13 = 117 is odd, 117 is in the sequence.
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MATHEMATICA
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Select[2Range[0, 9999] + 1, MemberQ[(DivisorSigma[1, #] * # &)/@Divisors[#], #] &] (* Alonso del Arte, Sep 18 2019 *)
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PROG
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(PARI) upto(n) = {my(res = List()); forstep(i = 1, sqrtnint(n, 4), 2, c = i^2*sigma(i^2); if(c <= n, listput(res, c))); listsort(res, 1); res}
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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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