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%I #19 Sep 08 2022 08:46:11
%S 3,5,17,26,257,65537,10866583226
%N Numbers n such that sigma(n) = 2*(phi(n-1)+1).
%C Subsequence of A256439. Supersequence of Fermat primes (A019434).
%C a(8) > 10^13. - _Giovanni Resta_, Jul 13 2015
%e 17 is in the sequence because sigma(17) = 18 = 2*(phi(16-1)+1) = 2*9.
%t Select[Range@ 100000, DivisorSigma[1, #] == 2 (EulerPhi[# - 1] + 1) &] (* _Michael De Vlieger_, Mar 31 2015 *)
%o (Magma) Set(Sort([n: n in [2..1000000] | SumOfDivisors(n) / (EulerPhi(n-1) + 1) eq 2 ]))
%o (PARI) first(m)={ my(v=vector(m),i,r);r=0;for(i=1,m,until(sigma(r)===2*(eulerphi(r-1)+1),r++);v[i]=r;print1(r,", "););v;} _Anders Hellström_, Jul 29 2015
%Y Cf. A000010, A000203, A256439.
%K nonn,more
%O 1,1
%A _Jaroslav Krizek_, Mar 31 2015
%E a(7) from _Giovanni Resta_, Jul 13 2015
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