This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A270416 #13 Apr 10 2016 10:20:33 %S A270416 3,5,6,10,17,34,40,60,85,136,204,240,4369,8224,8704,8738,10880,12336, %T A270416 13056,65537,131074,131584,139264,163840,164480,174760,208896,245760, %U A270416 262140,524296,526336,559232,835584,838848,2281736192,2694881440,2852170240,2863267840,3221274624,3233857728,4026593280 %N A270416 Numbers n such that sigma(n) - 1 and sigma(phi(n)) are both primes. %C A270416 Numbers n such that A039653(n) and A062402(n) are both primes. %C A270416 Intersection of A248792 and A062514. %C A270416 Prime terms are in A249759. %C A270416 Corresponding values of sigma(n) - 1: 3, 5, 11, 17, 17, 53, 89, 167, ... %C A270416 Corresponding values of sigma(phi(n)): 3, 7, 3, 7, 31, 31, 31, 31, 127, ... %e A270416 10 is in the sequence because sigma(10) - 1 = 18 - 1 = 17 and sigma(phi(10)) = sigma(4) = 7 (both primes). %t A270416 Select[Range[10^6], And[PrimeQ[DivisorSigma[1, #] - 1], PrimeQ@ DivisorSigma[1, EulerPhi@ #]] &] (* _Michael De Vlieger_, Mar 17 2016 *) %o A270416 (PARI) isok(n) = isprime(sigma(n)-1) && isprime(sigma(eulerphi(n))); \\ _Michel Marcus_, Mar 17 2016 %Y A270416 Cf. A000203, A039653, A062402, A062514, A248792, A249759. %K A270416 nonn %O A270416 1,1 %A A270416 _Jaroslav Krizek_, Mar 16 2016 %E A270416 a(35)-a(41) from _Giovanni Resta_, Apr 10 2016