login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A137600 Numbers n such that sigma(sigma(n))-phi(phi(n)) = 3n. 4

%I #6 Feb 25 2013 02:59:26

%S 18,32,36,144,576,36864,589824,2359296,9663676416

%N Numbers n such that sigma(sigma(n))-phi(phi(n)) = 3n.

%C If 2^p-1 is a Mersenne prime then n=9*2^(p-1) is in the sequence (the proof is easy). 18, 36, 144, 576, 36864, 589824 & 2359296 are such terms. 32 is in the sequence but it is not of this form.

%C a(10) > 10^10. - _Donovan Johnson_, Feb 25 2013

%t Do[If[DivisorSigma[1,DivisorSigma[1,n]]-EulerPhi[EulerPhi[n]]==3n, Print[n]],{n,25000000}]

%Y Cf. A000043, A000668.

%K more,nonn

%O 1,1

%A _Farideh Firoozbakht_, Feb 15 2008

%E a(9) from _Donovan Johnson_, Feb 25 2013

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified September 7 22:13 EDT 2024. Contains 375749 sequences. (Running on oeis4.)