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Search: id:A007620
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| A007620 |
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Practical numbers (second definition): numbers n such that every k <= n is a sum of proper divisors of n (for n>1).
(Formerly M4095)
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+0
4
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| 1, 6, 12, 18, 20, 24, 28, 30, 36, 40, 42, 48, 54, 56, 60, 66, 72, 78, 80, 84, 88, 90, 96, 100, 104, 108, 112, 120, 126, 132, 140, 144, 150, 156, 160, 162, 168, 176, 180, 192, 196, 198, 200, 204, 208, 210, 216, 220, 224, 228, 234, 240, 252, 260, 264, 270, 272, 276, 280, 288, 294, 300, 304, 306
(list; graph; listen)
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OFFSET
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1,2
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
R. Honsberger, Mathematical Gems, M.A.A., 1973, p. 113.
B. M. Stewart, Sums of distinct divisors, Amer. J. Math., 76 (1954), 779-785 quotes Srinivasan [Current Science (1948), p. 179] as using this definition. [R. J. Mathar]
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LINKS
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T. D. Noe, Table of n, a(n) for n=1..1000
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MATHEMATICA
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DeleteCases[ A005835, q_/; (Count[ CoefficientList[ Series[ Times@@( (1+z^#)& /@ Divisors[ q ] ), {z, 0, q} ], z ], 0 ]>0) ] - from wouter.meeussen(at)pandora.be
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CROSSREFS
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Cf. A005153 (first definition).
Adjacent sequences: A007617 A007618 A007619 this_sequence A007621 A007622 A007623
Sequence in context: A097216 A023196 A005835 this_sequence A100715 A094519 A088723
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KEYWORD
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nonn,nice,easy
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com), Robert G. Wilson v (rgwv(AT)rgwv.com)
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