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A138021
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a(n) = the number of positive divisors k of 2n where |k -2n/k| divides 2n.
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1
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2, 0, 2, 2, 0, 4, 0, 0, 2, 2, 0, 2, 0, 0, 2, 2, 0, 2, 0, 0, 2, 0, 0, 4, 2, 0, 2, 2, 0, 2, 0, 0, 0, 0, 0, 4, 0, 0, 0, 4, 0, 0, 0, 0, 4, 0, 0, 2, 2, 0, 0, 0, 0, 4, 2, 0, 0, 0, 0, 2, 0, 0, 0, 2, 0, 2, 0, 0, 0, 2, 0, 2, 0, 0, 4, 0, 0, 2, 0, 0, 2, 0, 0, 2, 0, 0, 0, 0, 0, 2, 2, 0, 0, 0, 0, 4, 0, 0, 0, 2, 0, 0, 0, 0, 2
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OFFSET
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1,1
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COMMENTS
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For every odd positive integer n, |k - n/k| divides n for 0 divisors of n.
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LINKS
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EXAMPLE
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The positive divisors of 12 are 1,2,3,4,6,12. Checking: |1- 12/1|=11 does not divide 12. |2- 12/2|=4 does divide 12. |3- 12/3|=1 does divide 12. |4- 12/4|=1 does divide 12. |6- 12/6|=4 does divide 12. And |12- 12/12|=11 does not divide 12. There are therefore four divisors k of 12 where |k -12/k| divides 12. So a(6) = 4.
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MAPLE
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A138021 := proc(n) local a, k ; a := 0 ; for k in numtheory[divisors](2*n) do if k-2*n/k <> 0 then if (2*n) mod abs(k-2*n/k) = 0 then a := a+1 ; fi ; fi ; od: a; end: seq(A138021(n), n=1..120) ; # R. J. Mathar, May 22 2008
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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