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 A366663 #13 Jan 07 2024 13:34:19
%S A366663 4,32,288,2560,26400,165888,2384928,15728640,141087744,1246080000,
%T A366663 14758128000,85996339200,1270928131200,8810420097024,70207948800000,
%U A366663 677066362060800,8218041445152000,43129128265187328,674757689572915200,4238841176064000000
%N A366663 a(n) = phi(9^n-1), where phi is Euler's totient function (A000010).
%H A366663 Max Alekseyev, <a href="/A366663/b366663.txt">Table of n, a(n) for n = 1..690</a>
%F A366663 a(n) = A295500(2*n) = 2 * A295500(n) * A366579(n). - _Max Alekseyev_, Jan 07 2024
%t A366663 EulerPhi[9^Range[30] - 1]
%o A366663 (PARI) {a(n) = eulerphi(9^n-1)}
%Y A366663 phi(k^n-1): A053287 (k=2), A295500 (k=3), A295501 (k=4), A295502 (k=5), A366623 (k=6), A366635 (k=7), A366654 (k=8), this sequence (k=9), A295503 (k=10), A366685 (k=11), A366711 (k=12).
%Y A366663 Cf. A000010, A024101, A057952, A366660, A366661, A366662, A366667.
%K A366663 nonn
%O A366663 1,1
%A A366663 _Sean A. Irvine_, Oct 15 2023