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A135239 Numbers m such that sigma(phi(m)) = 2*m. 1

%I #15 Oct 13 2023 07:51:25

%S 1209,27474741,55385589,222850677

%N Numbers m such that sigma(phi(m)) = 2*m.

%C From _Max Alekseyev_, Oct 12 2023: (Start)

%C No other terms below 3*10^11.

%C If a term m is odd, then phi(m) belongs to A001229; otherwise phi(m) belongs to A135240. (End)

%e sigma(phi(222850677)) = sigma(118879488) = 445701354 = 2*222850677, so 222850677 is in the sequence.

%t Do[If[DivisorSigma[1,EulerPhi[n]]==2n,Print[n]],{n,420000000}]

%Y Cf. A001229, A018784, A135240.

%K more,nonn

%O 1,1

%A _Farideh Firoozbakht_, Dec 26 2007

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Last modified July 13 04:46 EDT 2024. Contains 374267 sequences. (Running on oeis4.)