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 A136540 Numbers n such that sigma(n) = 7*phi(n). 7
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 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). LINKS Donovan Johnson, Table of n, a(n) for n = 1..1000 EXAMPLE sigma(12) = 28 = 7*phi(12) so 12 is in the sequence. MATHEMATICA Do[If[DivisorSigma[1, n]==7*EulerPhi[n], Print[n]], {n, 600000}] PROG (PARI) is(n)=sigma(n)==7*eulerphi(n) \\ Charles R Greathouse IV, May 09 2013 CROSSREFS Cf. A000043, A000668, A068390, A104900-A104904. Sequence in context: A026974 A210695 A109711 * A139612 A162629 A008504 Adjacent sequences:  A136537 A136538 A136539 * A136541 A136542 A136543 KEYWORD easy,nonn AUTHOR Farideh Firoozbakht, Jan 05 2008 STATUS approved

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