OFFSET
1,1
COMMENTS
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.
a(10) > 10^10. - Donovan Johnson, Feb 25 2013
MATHEMATICA
Do[If[DivisorSigma[1, DivisorSigma[1, n]]-EulerPhi[EulerPhi[n]]==3n, Print[n]], {n, 25000000}]
CROSSREFS
KEYWORD
more,nonn
AUTHOR
Farideh Firoozbakht, Feb 15 2008
EXTENSIONS
a(9) from Donovan Johnson, Feb 25 2013
STATUS
approved