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A115910 Numbers n such that phi(n) times n is a triangular number. 5
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..33.

EXAMPLE

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

MATHEMATICA

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

PROG

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

CROSSREFS

Cf. A115905, A115909.

Sequence in context: A074552 A193658 A195885 * A106569 A260769 A026032

Adjacent sequences:  A115907 A115908 A115909 * A115911 A115912 A115913

KEYWORD

nonn

AUTHOR

Giovanni Resta, Feb 06 2006

STATUS

approved

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Last modified March 29 05:39 EDT 2020. Contains 333105 sequences. (Running on oeis4.)