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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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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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njas, Robert G. Wilson v (rgwv(AT)rgwv.com)
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