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A080225 Number of perfect divisors of n. 7
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; text; internal format)
OFFSET

1,84

COMMENTS

Number of divisors d of n with sigma(d)=2*d (sigma=A000203); A080224(n)+a(n)+A080226(n)=A000005(n).

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Eric Weisstein's World of Mathematics, Perfect Number.

Wikipedia, Perfect number

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.

PROG

(Haskell)

a080225 n = length [d | d <- takeWhile (<= n) a000396_list, mod n d == 0]

-- Reinhard Zumkeller, Jan 20 2012

CROSSREFS

Cf. A000396, A000203, A147645.

Sequence in context: A317576 A070205 A138363 * A122841 A060862 A325801

Adjacent sequences:  A080222 A080223 A080224 * A080226 A080227 A080228

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller, Feb 07 2003

STATUS

approved

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Last modified June 20 08:06 EDT 2019. Contains 324229 sequences. (Running on oeis4.)