%I #6 Feb 25 2013 02:59:26
%S 18,32,36,144,576,36864,589824,2359296,9663676416
%N Numbers n such that sigma(sigma(n))phi(phi(n)) = 3n.
%C If 2^p1 is a Mersenne prime then n=9*2^(p1) is in the sequence (the proof is easy). 18, 36, 144, 576, 36864, 589824 & 2359296 are such terms. 32 is in the sequence but it is not of this form.
%C a(10) > 10^10.  _Donovan Johnson_, Feb 25 2013
%t Do[If[DivisorSigma[1,DivisorSigma[1,n]]EulerPhi[EulerPhi[n]]==3n, Print[n]],{n,25000000}]
%Y Cf. A000043, A000668.
%K more,nonn
%O 1,1
%A _Farideh Firoozbakht_, Feb 15 2008
%E a(9) from _Donovan Johnson_, Feb 25 2013
