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 A256444 #24 Jul 16 2025 20:40:25
%S A256444 3,5,17,26,257,65537,10866583226
%N A256444 Numbers k such that sigma(k) = 2*(phi(k-1)+1).
%C A256444 Subsequence of A256439. Supersequence of Fermat primes (A019434).
%C A256444 a(8) > 10^13. - _Giovanni Resta_, Jul 13 2015
%e A256444 17 is in the sequence because sigma(17) = 18 = 2*(phi(16-1)+1) = 2*9.
%t A256444 Select[Range@ 100000, DivisorSigma[1, #] == 2 (EulerPhi[# - 1] + 1) &] (* _Michael De Vlieger_, Mar 31 2015 *)
%o A256444 (Magma) Set(Sort([n: n in [2..1000000] | SumOfDivisors(n) / (EulerPhi(n-1) + 1) eq 2 ]));
%o A256444 (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 A256444 Cf. A000010, A000203, A256439.
%K A256444 nonn,more
%O A256444 1,1
%A A256444 _Jaroslav Krizek_, Mar 31 2015
%E A256444 a(7) from _Giovanni Resta_, Jul 13 2015