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A027687
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4-perfect (quadruply-perfect or sous-triple) numbers: sum of divisors of n is 4n.
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15
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OFFSET
| 1,1
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COMMENTS
| Contribution from Daniel Forgues (squid(AT)zensearch.com), May 09 2010: (Start)
Example:
30240 = 2^5*3^3*5*7
sigma(30240) = (2^6-1)/1*(3^4-1)/2*(5^2-1)/4*(7^2-1)/6
= (63)*(40)*(6)*(8)
= (7*3^2)*(2^3*5)*(2*3)*(2^3)
= 2^7*3^3*5*7
= (2^2) * (2^5*3^3*5*7)
= 4 * 30240 (End)
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REFERENCES
| R. K. Guy, Unsolved Problems in Number Theory, B2.
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LINKS
| T. D. Noe, Table of n, a(n) for n=1..36 (complete sequence)
Walter Nissen, Abundancy : Some Resources
Achim Flammenkamp, The Multiply Perfect Numbers Page
Fred Helenius, Link to Glossary and Lists
Eric Weisstein's World of Mathematics, Multiperfect Number.
Eric Weisstein's World of Mathematics, Sous-Triple.
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MATHEMATICA
| AbundantQ[n_]:=DivisorSigma[1, n]==4*n; a={}; Do[If[AbundantQ[n], AppendTo[a, n]], {n, 10^6}]; a [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Aug 16 2008]
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CROSSREFS
| Cf. A007539, A000396, A005820, A046060, A046061.
Sequence in context: A027665 A202598 A113286 * A190475 A109485 A156429
Adjacent sequences: A027684 A027685 A027686 * A027688 A027689 A027690
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KEYWORD
| nonn,fini
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AUTHOR
| Jean-Yves Perrier (nperrj(AT)ascom.ch)
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EXTENSIONS
| 4 more terms from Labos E. (labos(AT)ana.sote.hu)
Edited by Daniel Forgues (squid(AT)zensearch.com), May 11 2010
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