%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