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

1,2

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