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 A366578 #13 Jan 06 2024 12:17:11
%S A366578 3,7,18,56,126,434,1332,3836,10476,42560,109926,315112,816732,2790074,
%T A366578 8906760,30220288,64570086,229156928,706911048,2034690952,5357742012,
%U A366578 21838961760,56496274632,164750562956,456919958880,1517043139136,4661686010664,16489453890560
%N A366578 Sum of the divisors of 3^n+1.
%H A366578 Max Alekseyev, <a href="/A366578/b366578.txt">Table of n, a(n) for n = 0..691</a>
%F A366578 a(n) = sigma(3^n+1) = A000203(A034472(n)).
%e A366578 a(4)=126 because 3^4+1 has divisors {1, 2, 41, 82}.
%p A366578 a:=n->numtheory[sigma](3^n+1):
%p A366578 seq(a(n), n=0..100);
%t A366578 DivisorSigma[1,3^Range[0,30]+1] (* _Paolo Xausa_, Oct 15 2023 *)
%Y A366578 Cf. A000203, A002592, A024023, A057935, A057941, A074476, A075708, A274909, A366576, A366577, A366579, A366580
%K A366578 nonn
%O A366578 0,1
%A A366578 _Sean A. Irvine_, Oct 13 2023