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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 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.)