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%I #13 Apr 10 2016 10:20:33
%S 3,5,6,10,17,34,40,60,85,136,204,240,4369,8224,8704,8738,10880,12336,
%T 13056,65537,131074,131584,139264,163840,164480,174760,208896,245760,
%U 262140,524296,526336,559232,835584,838848,2281736192,2694881440,2852170240,2863267840,3221274624,3233857728,4026593280
%N Numbers n such that sigma(n) - 1 and sigma(phi(n)) are both primes.
%C Numbers n such that A039653(n) and A062402(n) are both primes.
%C Intersection of A248792 and A062514.
%C Prime terms are in A249759.
%C Corresponding values of sigma(n) - 1: 3, 5, 11, 17, 17, 53, 89, 167, ...
%C Corresponding values of sigma(phi(n)): 3, 7, 3, 7, 31, 31, 31, 31, 127, ...
%e 10 is in the sequence because sigma(10) - 1 = 18 - 1 = 17 and sigma(phi(10)) = sigma(4) = 7 (both primes).
%t Select[Range[10^6], And[PrimeQ[DivisorSigma[1, #] - 1], PrimeQ@ DivisorSigma[1, EulerPhi@ #]] &] (* _Michael De Vlieger_, Mar 17 2016 *)
%o (PARI) isok(n) = isprime(sigma(n)-1) && isprime(sigma(eulerphi(n))); \\ _Michel Marcus_, Mar 17 2016
%Y Cf. A000203, A039653, A062402, A062514, A248792, A249759.
%K nonn
%O 1,1
%A _Jaroslav Krizek_, Mar 16 2016
%E a(35)-a(41) from _Giovanni Resta_, Apr 10 2016
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