A135240 Numbers n such that phi(sigma(n))=2n.

36, 20575296

Offset

1,1

Comments

No other terms below 10^10. - Max Alekseyev, Sep 25 2009

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

Links

Table of n, a(n) for n=1..2.

Example

phi(sigma(20575296))=phi(71238999)=41150592=2*20575296, so 20575296 is in the sequence.

Mathematica

Do[If[EulerPhi[DivisorSigma[1, n]]==2n, Print[n]], {n, 350000000}]

Crossrefs

Cf. A071525, A135238, A135239.

Sequence in context: A071226 A134369 A151654 * A252092 A307949 A058432

Adjacent sequences: A135237 A135238 A135239 * A135241 A135242 A135243

Keyword

bref,more,nonn

Author

Farideh Firoozbakht, Dec 26 2007

Status

approved