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A115911
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Numbers k such that phi(k)*sigma(k) is a triangular number.
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1
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1, 2, 9, 11, 23, 118, 373, 556, 1332, 2420, 3081, 5251, 7642, 12671, 116836, 127627, 135861, 172676, 198912, 365408, 421902, 426710, 901273, 921736, 954068, 1001396, 1003333, 1006567, 1077452, 1161754, 1162514, 1364225, 1632361
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OFFSET
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1,2
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LINKS
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EXAMPLE
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phi(1003333)*sigma(1003333) = 1003888529280 = T(1416960).
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MATHEMATICA
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Select[Range[10^4], IntegerQ @ Sqrt[8 * EulerPhi[#] * DivisorSigma[1, #] + 1] &] (* Amiram Eldar, Sep 16 2019 *)
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PROG
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(PARI) isok(n) = ispolygonal(eulerphi(n)*sigma(n), 3); \\ Michel Marcus, Jan 09 2014
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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