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 A256439 #27 Jul 16 2025 20:45:48
%S A256439 3,5,17,26,171,257,265,1921,9385,26665,65537,263041,437761,1057801,
%T A256439 2038648321,10866583226,11453097097,982923711145
%N A256439 Numbers k such that phi(k-1)+1 divides sigma(k).
%C A256439 Numbers k such that A000010(k-1)+1 divides A000203(k).
%C A256439 Supersequence of Fermat primes (A019434).
%C A256439 Supersequence of A256444. Corresponding values of numbers k(n) = sigma(n) / (phi(n-1)+1) : 2, 2, 2, 2, 4, 2, 4, 4, 4, 4, 2, 4, 4, 4, ... - _Jaroslav Krizek_, Mar 31 2015
%C A256439 a(19) > 10^13. - _Giovanni Resta_, Jul 13 2015
%e A256439 17 is in the sequence because phi(16) + 1 divides sigma(17); 9 divides 18.
%p A256439 with(numtheory): A256439:=n->`if`(sigma(n) mod (phi(n-1)+1) = 0, n, NULL): seq(A256439(n), n=2..10^5); # _Wesley Ivan Hurt_, Mar 29 2015
%t A256439 Select[Range@ 1000000, Mod[DivisorSigma[1, #], EulerPhi[# - 1] + 1] == 0 &] (* _Michael De Vlieger_, Mar 29 2015 *)
%o A256439 (Magma) [n: n in [2..1000000] | Denominator(SumOfDivisors(n) / (EulerPhi(n-1) + 1)) eq 1 ];
%o A256439 (PARI) lista(nn) = {for (n=2, nn, if (sigma(n) % (eulerphi(n-1)+1) == 0, print1(n, ", ")););} \\ _Michel Marcus_, Mar 29 2015
%Y A256439 Cf. A000010, A000203, A019434.
%K A256439 nonn,more
%O A256439 1,1
%A A256439 _Jaroslav Krizek_, Mar 29 2015
%E A256439 a(15)-a(18) from _Giovanni Resta_, Jul 13 2015