A115911 Numbers k such that phi(k)*sigma(k) is a triangular number.

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

Offset: 1

Author

Giovanni Resta, Feb 06 2006

Keywords

• nonn

Examples

			phi(1003333)*sigma(1003333) = 1003888529280 = T(1416960).
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..160

Crossrefs

Cf. A062354, A069066.

Programs

Mathematica

Select[Range[10^4], IntegerQ @ Sqrt[8 * EulerPhi[#] * DivisorSigma[1, #] + 1] &] (* Amiram Eldar, Sep 16 2019 *)

PARI

isok(n) = ispolygonal(eulerphi(n)*sigma(n), 3); \\ Michel Marcus, Jan 09 2014