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A076646 Numbers n such that sigma(n) * antisigma(n) is a perfect square, where antisigma(n) = sum of the non-divisors of n that are between 1 and n. 0
1, 2, 15, 20, 24, 28, 104, 224, 5065, 766192670 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..10.

EXAMPLE

sigma(15) * antisigma(15) = 24 * 96 = 48^2, so 15 is a term of the sequence.

MATHEMATICA

l = {}; Do[s = DivisorSigma[1, n]; a = (n (n + 1)/2) - s; If[IntegerQ[Sqrt[a*s]], l = Append[l, n]], {n, 1, 10^5}]; l

CROSSREFS

Sequence in context: A042685 A031022 A194542 * A076617 A091791 A281660

Adjacent sequences:  A076643 A076644 A076645 * A076647 A076648 A076649

KEYWORD

nonn

AUTHOR

Joseph L. Pe, Oct 23 2002

EXTENSIONS

a(10) from Donovan Johnson, Jan 18 2012

STATUS

approved

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Last modified June 1 01:43 EDT 2020. Contains 334758 sequences. (Running on oeis4.)