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A080225
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Number of perfect divisors of n.
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7
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0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,84
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COMMENTS
| Number of divisors d of n with sigma(d)=2*d (sigma=A000203); A080224(n)+a(n)+A080226(n)=A000005(n).
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LINKS
| Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Perfect Number.
Wikipedia, Perfect number
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EXAMPLE
| Divisors of n=84: {1,2,3,4,6,7,12,14,21,24,28,42}, two of them are perfect: 6=A000396(1) and 28=A000396(2), therefore a(84)=2.
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PROG
| (Haskell)
a080225 n = length [d | d <- takeWhile (<= n) a000396_list, mod n d == 0]
-- Reinhard Zumkeller, Jan 20 2012
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CROSSREFS
| Cf. A000396, A000203, A147645.
Sequence in context: A028605 A070205 A138363 * A122841 A060862 A066087
Adjacent sequences: A080222 A080223 A080224 * A080226 A080227 A080228
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KEYWORD
| nonn
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AUTHOR
| Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Feb 07 2003
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