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 A366662 #10 Jan 07 2024 15:32:25
%S A366662 15,186,1680,15876,123690,1541568,8992680,111757968,967814400,
%T A366662 9366647892,62424587520,852903426816,4766016364260,55176998178240,
%U A366662 550081165885440,4829754617483040,31725040326819840,471309320999516160,2535353780263288800,33995669076586206864
%N A366662 Sum of the divisors of 9^n-1.
%H A366662 Max Alekseyev, <a href="/A366662/b366662.txt">Table of n, a(n) for n = 1..690</a>
%F A366662 a(n) = sigma(9^n-1) = A000203(A024101(n)).
%F A366662 a(n) = A366576(2*n) = A366576(n) * A366578(n) * (2^(4 + A007814(n)) - 1) / (2^(3 + A007814(n)) - 1) / 3. - _Max Alekseyev_, Jan 07 2024
%e A366662 a(2)=186 because 9^2-1 has divisors {1, 2, 4, 5, 8, 10, 16, 20, 40, 80}.
%p A366662 a:=n->numtheory[sigma](9^n-1):
%p A366662 seq(a(n), n=1..30);
%t A366662 DivisorSigma[1, 9^Range[30]-1]
%Y A366662 Cf. A024101, A000203, A366660, A366661, A366663.
%Y A366662 Cf. A075708, A366576, A366603, A366613, A366622, A366634, A366653, A102146, A366684, A366710.
%K A366662 nonn
%O A366662 1,1
%A A366662 _Sean A. Irvine_, Oct 15 2023