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%I #7 Sep 13 2018 09:31:36
%S 36,20575296
%N Numbers n such that phi(sigma(n))=2n.
%C No other terms below 10^10. - _Max Alekseyev_, Sep 25 2009
%C 6.5*10^12 < a(3) <= 180577942272000000. We also have phi(sigma(n)) = 3*n for n = 144982963520227123200 and phi(sigma(n)) = 4*n for n = 16310583396025551360000 thus these may not be the smallest such n. - _Giovanni Resta_, Sep 13 2018
%e phi(sigma(20575296))=phi(71238999)=41150592=2*20575296, so 20575296 is in the sequence.
%t Do[If[EulerPhi[DivisorSigma[1,n]]==2n,Print[n]],{n,350000000}]
%Y Cf. A071525, A135238, A135239.
%K bref,more,nonn
%O 1,1
%A _Farideh Firoozbakht_, Dec 26 2007
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