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