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A136540
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Numbers n such that sigma(n) = 7*phi(n).
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7
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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, 616722, 646646, 785118, 965960, 1027000
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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 greater than 3 then m = 65*2^(p-2)*(2^p-1) is in the sequence (the proof is easy).
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LINKS
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Donovan Johnson, Table of n, a(n) for n = 1..1000
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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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PROG
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(PARI) is(n)=sigma(n)==7*eulerphi(n) \\ Charles R Greathouse IV, May 09 2013
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CROSSREFS
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Cf. A000043, A000668, A068390, A104900-A104904.
Sequence in context: A026974 A210695 A109711 * A139612 A162629 A008504
Adjacent sequences: A136537 A136538 A136539 * A136541 A136542 A136543
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KEYWORD
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easy,nonn
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AUTHOR
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Farideh Firoozbakht, Jan 05 2008
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STATUS
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approved
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