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A228061
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Numbers n such that n^2 = sigma(m) for some m.
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4
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1, 2, 6, 11, 12, 16, 18, 20, 24, 28, 30, 31, 32, 36, 40, 42, 44, 48, 52, 54, 56, 60, 62, 64, 66, 68, 70, 72, 76, 78, 80, 84, 88, 90, 96, 98, 100, 102, 104, 108, 112, 114, 120, 124, 126, 128, 132, 136, 140, 144, 150, 152, 154, 156, 160, 162, 164, 168, 172, 174
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OFFSET
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1,2
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LINKS
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T. D. Noe, Table of n, a(n) for n = 1..1000
P. Pollack and C. Pomerance, Square values of Euler's function, submitted for publication (2013). (See section 5.)
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FORMULA
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a(n) = sqrt(A038688(n)).
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MATHEMATICA
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nn = 40000; t = Select[Union[DivisorSigma[1, Range[nn]]], IntegerQ[Sqrt[#]] &]; t = Sqrt[t]; t = Select[t, # < Sqrt[nn] &]
With[{nn=50000}, Union[Select[Sqrt[DivisorSigma[1, Range[nn]]], IntegerQ[ #] && #<=Sqrt[nn]&]]] (* Harvey P. Dale, Jul 12 2021 *)
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CROSSREFS
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Cf. A038688 (square of these numbers).
Cf. A221284 (similar numbers for the phi function).
Sequence in context: A274689 A123112 A092189 * A191772 A323044 A057976
Adjacent sequences: A228058 A228059 A228060 * A228062 A228063 A228064
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KEYWORD
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nonn
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AUTHOR
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T. D. Noe, Sep 04 2013
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STATUS
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approved
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