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A137600
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Numbers n such that sigma(sigma(n))-phi(phi(n)) = 3n.
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4
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OFFSET
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1,1
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COMMENTS
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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.
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LINKS
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MATHEMATICA
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Do[If[DivisorSigma[1, DivisorSigma[1, n]]-EulerPhi[EulerPhi[n]]==3n, Print[n]], {n, 25000000}]
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CROSSREFS
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KEYWORD
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more,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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