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Search: id:A136540
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| A136540 |
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Numbers n such that sigma(n)=7*phi(n). |
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+0
2
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| 12, 78, 140, 910, 2214, 4180, 4674, 8008, 16120, 25758, 27170, 46816, 54530, 58302, 94240, 99484, 116116, 200260, 233740, 257140, 264160, 350740, 371898, 383656, 479864, 518022, 523218, 551540, 561340, 575598
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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If 2^p-1 is a Mersenne prime greater than 3 then m=65*2^(p-2)*(2^p-1) is in the sequence (the proof is easy).
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EXAMPLE
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sigma(12)=28=7*phi(12) so 12 is in the sequence.
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MATHEMATICA
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Do[If[DivisorSigma[1, n]==7*EulerPhi[n], Print[n]], {n, 600000}]
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CROSSREFS
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Cf. A000043, A000668, A068390, A104900-A104904.
Adjacent sequences: A136537 A136538 A136539 this_sequence A136541 A136542 A136543
Sequence in context: A026964 A026974 A109711 this_sequence A139612 A008504 A008494
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KEYWORD
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easy,nonn
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AUTHOR
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Farideh Firoozbakht (mymontain(AT)yahoo.com), Jan 05 2008
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