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 A136547 Numbers n such that sigma(n) = 5*phi(n). 6
 56, 190, 812, 1672, 4522, 5278, 16065, 24244, 25070, 33915, 39585, 56252, 80104, 93496, 102856, 107156, 140296, 157586, 220616, 224536, 316274, 317205, 365638, 389732, 423045, 479655, 546592, 559845, 596666, 601312, 696514, 731962, 1123605, 1161508, 1181895 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS If p>2 and 2^p-1 is prime (a Mersenne prime) then 377*2^(p-2)*(2^p-1) is in the sequence (the proof is easy). So for n>1 377*2^(A000043(n)-2)*(2^A000043(n)-1) is in the sequence. LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 (calculated using data from Jud McCranie, terms 1..1000 from Donovan Johnson) Kevin A. Broughan and Daniel Delbourgo, On the Ratio of the Sum of Divisors and Euler’s Totient Function I, Journal of Integer Sequences, Vol. 16 (2013), Article 13.8.8. Kevin A. Broughan and Qizhi Zhou, On the Ratio of the Sum of Divisors and Euler's Totient Function II, Journal of Integer Sequences, Vol. 17 (2014), Article 14.9.2. EXAMPLE sigma(56) = 120 = 5*24 = 5*phi(56) so 56 is in the sequence. MATHEMATICA Do[If[DivisorSigma[1, m]==5*EulerPhi[m], Print[m]], {m, 1500000}] PROG (PARI) is(n)=sigma(n)==5*eulerphi(n) \\ Charles R Greathouse IV, May 09 2013 CROSSREFS Cf. A000043, A062699, A104900, A104901, A104902. Sequence in context: A224108 A234114 A234107 * A264303 A200833 A241611 Adjacent sequences:  A136544 A136545 A136546 * A136548 A136549 A136550 KEYWORD nonn AUTHOR Farideh Firoozbakht, Jan 29 2008, Jan 30 2008 STATUS approved

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Last modified October 27 22:24 EDT 2021. Contains 348305 sequences. (Running on oeis4.)