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 A366576 #11 Jan 05 2024 19:09:16
%S A366576 3,15,42,186,399,1680,3282,15876,31836,123690,277344,1541568,2391486,
%T A366576 8992680,25483332,111757968,193819392,967814400,1744488660,9366647892,
%U A366576 16912999320,62424587520,144219337920,852903426816,1397135488896,4766016364260,12477973754400
%N A366576 Sum of the divisors of 3^n-1.
%H A366576 Max Alekseyev, <a href="/A366576/b366576.txt">Table of n, a(n) for n = 1..690</a>
%F A366576 a(n) = sigma(3^n-1) = A000203(A024023).
%e A366576 a(4)=186 because 3^4-1 has divisors {1, 2, 4, 5, 8, 10, 16, 20, 40, 80}.
%p A366576 a:=n->numtheory[sigma](3^n-1):
%p A366576 seq(a(n), n=1..100);
%t A366576 DivisorSigma[1,3^Range[30]-1] (* _Paolo Xausa_, Oct 15 2023 *)
%Y A366576 Cf. A024023, A000203, A075708, A133801, A295500, A366575.
%Y A366576 Cf. A075708, A366603, A366613, A366622, A366634, A366653, A366662, A102146, A366684, A366710.
%K A366576 nonn
%O A366576 1,1
%A A366576 _Sean A. Irvine_, Oct 13 2023