A115910 Numbers k such that phi(k)*k is a triangular number.

1, 3, 15, 84, 217, 255, 1710, 2967, 5363, 11242, 24066, 27370, 29697, 29953, 60977, 65535, 69324, 103257, 159761, 209305, 228333, 446516, 598559, 615410, 691410, 1231305, 1238358, 1365175, 1467732, 1841154, 1966220, 2081070, 2562370

Offset: 1

Author

Giovanni Resta, Feb 06 2006

Keywords

• nonn

Examples

			phi(24066)*24066 = 164611440 = T(18144).
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..100

Crossrefs

Cf. A002618, A000217, A115905, A115909.

Programs

Mathematica

Select[Range[2.6*10^6],OddQ[Sqrt[8#*EulerPhi[#]+1]]&] (* Harvey P. Dale, Jul 11 2017 *)

PARI

isok(n) = ispolygonal(n*eulerphi(n), 3); \\ Michel Marcus, Jan 25 2014