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 A278919 #18 Sep 22 2024 17:48:45 %S A278919 3,4,5,17,26,257,65537,4294967297 %N A278919 Numbers n such that phi(n-2) divides sigma(n-1)+1. %C A278919 Numbers n such that A000010(n-2) divides A000203(n-1)+1. %C A278919 Supersequence of Fermat primes (A019434). %C A278919 Conjecture: this sequence is finite. %C A278919 Any further terms are > 10^12. - _Lucas A. Brown_, Sep 22 2024 %H A278919 Lucas A. Brown, <a href="https://github.com/lucasaugustus/oeis/blob/main/A278919.py">Python program</a>. %e A278919 3 is in this sequence because phi(1) divides sigma(2)+1; 1 divides 4. %e A278919 4 is in this sequence because phi(2) divides sigma(3)+1; 1 divides 5. %e A278919 5 is in this sequence because phi(3) divides sigma(4)+1; 2 divides 8. %e A278919 17 is in this sequence because phi(15) divides sigma(16)+1; 8 divides 32. %t A278919 Select[Range[3,66000],Divisible[DivisorSigma[1,(#-1)]+1,EulerPhi[#-2]]&] (* _Ivan N. Ianakiev_, Dec 05 2016 *) %o A278919 (Magma) [3] cat [n: n in [4..10000000] | Denominator((SumOfDivisors(n-1)+1)/EulerPhi(n-2)) eq 1]; %Y A278919 Cf. A000010, A000203, A019434, A256439. %K A278919 nonn,more,hard %O A278919 1,1 %A A278919 _Juri-Stepan Gerasimov_, Nov 30 2016 %E A278919 a(8) from _Ivan N. Ianakiev_, Dec 05 2016