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 A068391 Numbers n such that sigma(n) = 3*phi(n). 7
 2, 15, 357, 3339, 5049, 10659, 12441, 24969, 99693, 124355, 132957, 145145, 353133, 423657, 596037, 655707, 734517, 745503, 894387, 1406427, 1641783, 1823877, 1936557, 3295047, 4108401, 4194183, 4776201, 5574699, 5842137, 5971251, 6132789, 6953765, 7649915 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS From Farideh Firoozbakht, May 01 2009: (Start) If m>1 and 2*3^m-1 is prime then n=7*3^(m-1)*(2*3^m-1) is in the sequence. Because sigma(n)=8*(3^m-1)/2*(2*3^m)=8*3^m*(3^m-1)=3*6*(2*3^(m-2))*(2*3^m-2) =3*phi(7)*phi(3^(m-1))*phi(2*3^m-1))=3*phi(7*3^(m-1)*(2*3^m-1))=3*phi(n). (End) 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 Eulerâ€™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. MATHEMATICA Select[Range[765*10^4], DivisorSigma[1, #]==3EulerPhi[#]&] (* Harvey P. Dale, Aug 25 2019 *) PROG (PARI) for(n=1, 500000, if(sigma(n)==3*eulerphi(n), print1(n, ", "))) CROSSREFS Cf. A000203. Subsequence of A087943 (sigma(k) is a multiple of 3). Sequence in context: A297077 A175563 A231443 * A215743 A254224 A071102 Adjacent sequences:  A068388 A068389 A068390 * A068392 A068393 A068394 KEYWORD easy,nonn AUTHOR Benoit Cloitre, Mar 03 2002 EXTENSIONS More terms from Rick L. Shepherd, May 14 2002 STATUS approved

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Last modified May 15 17:11 EDT 2021. Contains 343920 sequences. (Running on oeis4.)