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%I #29 Aug 23 2018 10:25:21
%S 12,70,88,1888,4030,5830,32128,521728,1848964,8378368,34359083008,
%T 66072609790,549753192448,259708613909470,2251799645913088
%N Numbers n whose abundance is 4: sigma(n) - 2n = 4.
%C A subset of A045769.
%C If 2^m-5 is prime then n=2^(m-1)*(2^m-5) is in the sequence (see comment lines of the sequence A088831). 12, 88, 1888, 32128, 521728, 8378368 & 34359083008 are such terms. - _Farideh Firoozbakht_, Feb 15 2008
%C a(14) > 10^12. - _Donovan Johnson_, Dec 08 2011
%C a(14) > 10^13. - _Giovanni Resta_, Mar 29 2013
%C a(16) > 10^18. - _Hiroaki Yamanouchi_, Aug 23 2018
%C Any term x of this sequence can be combined with any term y of A125246 to satisfy the property (sigma(x)+sigma(y))/(x+y) = 2, which is a necessary (but not sufficient) condition for two numbers to be amicable. - _Timothy L. Tiffin_, Sep 13 2016
%F Solutions to sigma[x]-2x=4
%e Abundances of terms in A045769: {-5,4,4,4,4,4,4,4,4,4} so A045769[1]=9 is not here.
%t Do[If[DivisorSigma[1,n]==2n+4,Print[n]],{n,650000000}] - _Farideh Firoozbakht_, Feb 15 2008
%o (PARI) is(n)=sigma(n)==2*n+4 \\ _Charles R Greathouse IV_, Feb 21 2017
%Y Cf. A033880, A045768, A045769, A088830, A059608, A125246 (deficiency 4).
%K nonn,more
%O 1,1
%A _Labos Elemer_, Oct 28 2003
%E One more terms from _Farideh Firoozbakht_, Feb 15 2008
%E a(11)-a(12) from _Donovan Johnson_, Dec 23 2008
%E a(13) from _Donovan Johnson_, Dec 08 2011
%E a(14)-a(15) from _Hiroaki Yamanouchi_, Aug 23 2018
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