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Numbers k such that k = phi(sigma(phi(sigma(phi(sigma(k)))))).
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%I #14 May 19 2022 09:15:08

%S 1,2,8,12,16,24,30,128,240,720,1200,1536,1860,6912,15552,15840,18144,

%T 32768,67392,83160,92160,142560,272160,290304,290400,712800,1140480,

%U 1190400,3345408,3571200,5702400,14859936,29719872,50319360,118879488,2147483648,3889036800

%N Numbers k such that k = phi(sigma(phi(sigma(phi(sigma(k)))))).

%t Select[Range[3*10^7],EulerPhi[DivisorSigma[1,EulerPhi[DivisorSigma[1, EulerPhi[ DivisorSigma[1,#]]]]]]==#&] (* _Harvey P. Dale_, Apr 02 2015 *)

%Y Cf. A000010, A000203.

%K easy,nonn

%O 1,2

%A _Benoit Cloitre_, Mar 02 2002

%E More terms from _Robert Gerbicz_, Sep 05 2002

%E a(34)-a(37) from _Amiram Eldar_, May 19 2022