
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

