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A046762
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Numbers k such that the sum of the squares of the divisors of k is divisible by k.
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11
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1, 10, 60, 65, 84, 130, 140, 150, 175, 260, 350, 420, 525, 780, 1050, 1105, 1820, 2100, 2210, 4420, 4650, 5425, 5460, 8840, 10500, 10850, 13260, 16275, 19720, 20150, 20737, 21700, 30225, 30940, 32045, 32550, 41474, 45500, 55250, 57350, 60450
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OFFSET
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1,2
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COMMENTS
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Compare with multiply perfect numbers A007691. Here Sum(divisors) is replaced by Sum(square of divisors).
Problem 11090 proves that this sequence is infinite. - T. D. Noe, Apr 18 2006
Cai, Chen, & Zhang prove that sigma_2(n)/n = b has only finitely many solutions for a given b, and hence (since this sequence is infinite) sigma_2(a(n))/a(n) is unbounded. - Charles R Greathouse IV, Jul 21 2016
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LINKS
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EXAMPLE
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k = 65 = a(4), sigma(2,65) = 4420 = 65*68 = 68*k;
k = 1820 = a(17), the divisor-square sum is 4641000 = 2550*1820 = 2550*k.
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MATHEMATICA
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Select[Range[70000], Divisible[DivisorSigma[2, #], #]&] (* Harvey P. Dale, Dec 15 2010 *)
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PROG
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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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