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 A137600 Numbers n such that sigma(sigma(n))-phi(phi(n)) = 3n. 4
 18, 32, 36, 144, 576, 36864, 589824, 2359296, 9663676416 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified August 9 13:53 EDT 2024. Contains 375042 sequences. (Running on oeis4.)