%I #10 Mar 25 2015 01:20:56
%S 1,4,5,15,18,30,36
%N Numbers n such that g(sigma(n)) = phi(n), where g(n) = prime(n+1) - prime(n).
%C No more terms below 10^6. - _Robert G. Wilson v_, Jan 05 2002
%C No more terms below 2 * 10^17. - _Charles R Greathouse IV_, Feb 15 2011
%C On the Riemann hypothesis, a(7) = 36 is the last member of this sequence. - _Charles R Greathouse IV_, Feb 15 2011
%e g(sigma(4)) = g(7) = prime(8)-prime(7) = 19-17 = 2 = phi(4).
%t g[x_] := Prime[x + 1] - Prime[x]; Select[Range[1, 10^5], g[DivisorSigma[1, # ]] == EulerPhi[ # ] &]
%K nonn,fini,full
%O 1,2
%A _Joseph L. Pe_, Jan 04 2002