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A115910 Numbers k such that phi(k)*k is a triangular number. 6
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
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
Sequence in context: A074552 A193658 A195885 * A355094 A106569 A260769
KEYWORD
nonn
AUTHOR
Giovanni Resta, Feb 06 2006
STATUS
approved

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Last modified March 1 18:30 EST 2024. Contains 370443 sequences. (Running on oeis4.)