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 A115911 Numbers k such that phi(k)*sigma(k) is a triangular number. 1
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Amiram Eldar, Table of n, a(n) for n = 1..160 EXAMPLE phi(1003333)*sigma(1003333) = 1003888529280 = T(1416960). MATHEMATICA Select[Range[10^4], IntegerQ @ Sqrt[8 * EulerPhi[#] * DivisorSigma[1, #] + 1] &] (* Amiram Eldar, Sep 16 2019 *) PROG (PARI) isok(n) = ispolygonal(eulerphi(n)*sigma(n), 3); \\ Michel Marcus, Jan 09 2014 CROSSREFS Cf. A062354, A069066. Sequence in context: A041581 A042463 A081613 * A065596 A280418 A042053 Adjacent sequences: A115908 A115909 A115910 * A115912 A115913 A115914 KEYWORD nonn AUTHOR Giovanni Resta, Feb 06 2006 STATUS approved

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Last modified February 27 16:21 EST 2024. Contains 370378 sequences. (Running on oeis4.)