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Numbers n such that sigma(n) = phi(prime(n)-1).
2

%I #13 Jul 08 2016 08:13:49

%S 1,7,19,47,115,177,280,297,340,406,470,930,1240,1608,1836,2232,2380,

%T 2484,4326,8360,33864,166152,216216,220248,299250,394500

%N Numbers n such that sigma(n) = phi(prime(n)-1).

%C No further term below 4*10^6.

%C No further terms below 10^8. - _Julien Kluge_, Jul 08 2016

%e sigma(19) = 20, prime(19) = 67, phi(66) = 20, hence 20 is a term.

%t Select[Range[1, 100000],

%t DivisorSigma[1, #] == EulerPhi[Prime[#] - 1] &] (* _Julien Kluge_, Jul 08 2016 *)

%o (PARI) m=400000;for(n=1,m, if(sigma(n)==eulerphi(prime(n)-1),print1(n,",")))

%Y Cf. A107815, A107816.

%K nonn,more

%O 1,2

%A _Benoit Cloitre_, Feb 23 2002

%E More terms from _Klaus Brockhaus_, Mar 02 2002, revised May 24 2005