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Numbers k such that phi(k) = sigma(phi(phi(k))).
1

%I #19 Sep 08 2022 08:45:23

%S 1,2,29,58,1119,1492,2238,20441,20803,40882,41606,550339,550687,

%T 552407,553339,557239,568487,573319,579367,581279,584039,589399,

%U 593959,644959,646303,653317,655879,669599,676837,823311,823527,826599,829233,830199,830961,835959

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

%H Hiroaki Yamanouchi, <a href="/A115448/b115448.txt">Table of n, a(n) for n = 1..387</a>

%t lst={};Do[u = EulerPhi@n; If[u == DivisorSigma[1, EulerPhi@u], AppendTo[lst, n]], {n, 10^6}];lst

%o (PARI)

%o for(n=1,10^7,if(eulerphi(n)==sigma(eulerphi(eulerphi(n))),print1(n,", "))) \\ _Derek Orr_, Sep 19 2014

%o (Magma) [n: n in [1..2*10^6] | EulerPhi(n) eq DivisorSigma(1,EulerPhi(EulerPhi(n)))]; // _Vincenzo Librandi_, Dec 25 2015

%K nonn

%O 1,2

%A _Giovanni Resta_, Jan 22 2006

%E a(35)-a(36) from _Hiroaki Yamanouchi_, Sep 19 2014