A137600 Numbers n such that sigma(sigma(n))-phi(phi(n)) = 3n.

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

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

Links

Table of n, a(n) for n=1..9.

Mathematica

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

Crossrefs

Cf. A000043, A000668.

Sequence in context: A305220 A316798 A070744 * A299707 A350958 A045556

Adjacent sequences: A137597 A137598 A137599 * A137601 A137602 A137603

Keyword

more,nonn

Author

Farideh Firoozbakht, Feb 15 2008

Extensions

a(9) from Donovan Johnson, Feb 25 2013

Status

approved