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 A273999 #24 Oct 06 2021 12:04:19 %S A273999 1,5,17,257,46657,65537,148997,67371265,405458497,1370776577, %T A273999 3497539601,4294967297,80542440001,422240040001,1911029760001, %U A273999 139251776898727937,286245437364810001,6017402415698251777,18446744073709551617 %N A273999 Numbers of the form n^2+1 that divide 4^n-1. %C A273999 Corresponding values of n are given by A273870(k)-1 for k>=1. %C A273999 Contains Fermat numbers (A000215) greater than 3. %C A273999 Also, numbers of the form n^2+1 that divide (4^k)^n-1 for all k >= 0. %C A273999 a(20) > 4*10^24, if it exists. - _Giovanni Resta_, Feb 26 2020 %F A273999 a(n) = (A273870(n)-1)^2+1. %e A273999 17 = 4^2+1 is a term because divides 4^4-1; 255 / 17 = 15. %o A273999 (PARI) is(n) = ceil(sqrt(n-1))==sqrtint(n-1) && Mod(4, n)^(sqrtint(n))==1 %o A273999 for(n=0, 1e12, if(is(n^2+1), print1(n^2+1, ", "))) \\ _Felix Fröhlich_, Jun 06 2016 %Y A273999 Subsequence of A002522 (numbers of the form n^2+1). %Y A273999 Prime terms are in A274000. %Y A273999 Cf. A000215, A002522, A273870, A273871, A274000. %K A273999 nonn,more %O A273999 1,2 %A A273999 _Jaroslav Krizek_, Jun 06 2016 %E A273999 a(16)-a(19) from _Lars Blomberg_, Aug 10 2016 %E A273999 Edited by _Max Alekseyev_, Apr 30 2018