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A067704 Numbers n such that phi(sigma(n)) = 2*phi(n). 4
2, 6, 8, 9, 24, 28, 70, 78, 128, 140, 222, 234, 280, 312, 350, 366, 384, 438, 496, 525, 666, 864, 888, 910, 918, 936, 942, 1036, 1098, 1158, 1232, 1314, 1400, 1464, 1575, 1662, 1708, 1752, 1824, 1836, 1878, 1900, 1938, 2044, 2382, 2480, 2526, 2590, 2664 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

M. F. Hasler and Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 (first 110 terms from Hasler)

MATHEMATICA

Select[Range[3000], EulerPhi[DivisorSigma[1, #]]==2EulerPhi[#]&] (* Harvey P. Dale, Oct 15 2020 *)

PROG

(PARI) is(n)=2*eulerphi(n=factor(n))==eulerphi(sigma(n)) \\ Charles R Greathouse IV, Nov 27 2013

CROSSREFS

Cf. A137815, A137816.

Sequence in context: A172209 A107737 A045138 * A284889 A192125 A166270

Adjacent sequences:  A067701 A067702 A067703 * A067705 A067706 A067707

KEYWORD

nonn

AUTHOR

Benoit Cloitre, Feb 05 2002

STATUS

approved

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Last modified May 16 08:07 EDT 2021. Contains 343940 sequences. (Running on oeis4.)