A067704 Numbers n such that phi(sigma(n)) = 2*phi(n).
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
Offset: 1
Keywords
Links
- M. F. Hasler and Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 (first 110 terms from Hasler)
Programs
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Mathematica
Select[Range[3000],EulerPhi[DivisorSigma[1,#]]==2EulerPhi[#]&] (* Harvey P. Dale, Oct 15 2020 *)
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PARI
is(n)=2*eulerphi(n=factor(n))==eulerphi(sigma(n)) \\ Charles R Greathouse IV, Nov 27 2013