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A277368
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Numbers such that the number of their divisors divide the sum of their aliquot parts.
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1
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1, 4, 10, 16, 25, 26, 34, 56, 58, 60, 64, 74, 81, 82, 90, 96, 100, 106, 120, 121, 122, 132, 146, 178, 184, 194, 202, 204, 216, 218, 226, 234, 248, 274, 276, 289, 298, 306, 312, 314, 346, 348, 362, 364, 376, 386, 394, 408, 440, 458, 466, 480, 482, 492, 504, 514
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OFFSET
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1,2
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COMMENTS
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If p is a prime such that p == 2 (mod 3) then p^2 is a term. Bateman et al. (1981) proved that the asymptotic density of this sequence is 0. - Amiram Eldar, Jan 16 2020
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REFERENCES
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Richard G. Guy, Unsolved Problems in Number Theory, 3rd ed., Springer, 2004, chapter 2, p. 76.
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LINKS
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Paul T. Bateman, Paul Erdős, Carl Pomerance and E.G. Straus, The arithmetic mean of the divisors of an integer, in Marvin I. Knopp (ed.), Analytic Number Theory, Proceedings of a Conference Held at Temple University, Philadelphia, May 12-15, 1980, Lecture Notes in Mathematics, Vol. 899, Springer, Berlin - New York, 1981, pp. 197-220, alternative link.
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FORMULA
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EXAMPLE
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sigma(26) - 26 = 42 - 26 = 16, d(26) = 4 and 16 / 4 = 4.
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MAPLE
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with(numtheory): P:= proc(q) local n; for n from 1 to q do
if type((sigma(n)-n)/tau(n), integer) then print(n); fi; od; end: P(10^3);
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MATHEMATICA
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Select[Range@ 520, Mod[DivisorSigma[1, #] - #, DivisorSigma[0, #]] == 0 &] (* Michael De Vlieger, Oct 14 2016 *)
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PROG
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(PARI) isok(n) = ((sigma(n) - n) % numdiv(n)) == 0; \\ Michel Marcus, Oct 11 2016
(Magma) [k:k in [1..550]| (DivisorSigma(1, k)-k) mod DivisorSigma(0, k) eq 0]; // Marius A. Burtea, Jan 16 2020
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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