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A135240 Numbers n such that phi(sigma(n))=2n. 2
36, 20575296 (list; graph; refs; listen; history; text; internal format)
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

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Last modified July 26 01:15 EDT 2021. Contains 346294 sequences. (Running on oeis4.)