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A062699 Numbers n such that sigma(n) = 2*phi(n).

%I A062699 #37 Dec 05 2019 04:05:38
%S A062699 3,35,1045,24871,29029,50065,58435,64285,87685,137885,140335,1390753,
%T A062699 1529983,1739507,2011009,2086903,3189625,3281663,3501605,3722875,
%U A062699 3830827,3852155,6605945,7711405,8409305,9815195,11413205,11569805,13321295,13932919,16540205
%N A062699 Numbers n such that sigma(n) = 2*phi(n).
%C A062699 3 is the only prime term of this sequence. There is no term of the form p^k where p is a prime and k>1. All terms are odd because if n is even then 2*phi(n)=phi(2n)<=n<sigma(n). - _Farideh Firoozbakht_, Apr 01 2005, Feb 24 2007
%H A062699 Amiram Eldar, <a href="/A062699/b062699.txt">Table of n, a(n) for n = 1..10000</a> (calculated using data from Jud McCranie, terms 1..50 from Harry J. Smith, terms 51..1000 from Donovan Johnson)
%H A062699 Kevin A. Broughan and Daniel Delbourgo, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL16/Broughan/broughan26.html">On the Ratio of the Sum of Divisors and Euler’s Totient Function I</a>, Journal of Integer Sequences, Vol. 16 (2013), Article 13.8.8.
%H A062699 Kevin A. Broughan and Qizhi Zhou, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL17/Broughan/bro32.html">On the Ratio of the Sum of Divisors and Euler's Totient Function II</a>, Journal of Integer Sequences, Vol. 17 (2014), Article 14.9.2.
%t A062699 Select[Range[10^6], DivisorSigma[1, #] == 2 * EulerPhi[#] &] (* _Amiram Eldar_, Dec 04 2019 *)
%o A062699 (PARI) for(n=1,500000, if(sigma(n)==eulerphi(n)*2,print(n)))
%o A062699 (PARI) n=0; for (m=1, 10^9, if(sigma(m)==2*eulerphi(m), write("b062699.txt", n++, " ", m); if (n==50, break)) ) \\ _Harry J. Smith_, Aug 09 2009
%o A062699 (PARI) is(n)=my(f=factor(n)); sigma(f)==2*eulerphi(f) \\ _Charles R Greathouse IV_, Aug 13 2015
%Y A062699 Cf. A068390, A104900, A104901.
%Y A062699 Subsequence of A028983 (sigma(k) is even).
%K A062699 nonn
%O A062699 1,1
%A A062699 _Jason Earls_, Jul 11 2001
%E A062699 More terms from _Labos Elemer_, Nov 23 2001