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%I #9 Nov 29 2019 04:11:14
%S 3,5,3193
%N Numbers n such that sigma(sigma(n))-phi(phi(n)) = 2n.
%C No other term <10^6.
%C No other terms below 5*10^6 [From Jasper Mulder (jasper.mulder(AT)planet.nl), Jun 03 2010]
%C No other terms < 10^10. - _Donovan Johnson_, Feb 25 2013
%C a(4) > 1.5*10^12, if it exists. - _Giovanni Resta_, Nov 29 2019
%e 3193 is in the sequence because sigma(sigma(3193))-phi(phi(3193)) = sigma(3328)-phi(3060) = 7154-768 = 6386 = 2*3193.
%p with(numtheory): a:=proc(n) if sigma(sigma(n))-phi(phi(n))=2*n then n else end if end proc: seq(a(n),n=1..1000000);
%Y Cf. A136131, A136132, A137600.
%K more,nonn,bref
%O 1,1
%A _Emeric Deutsch_, Mar 03 2008
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