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 A366579 #15 Jan 06 2024 12:22:59
%S A366579 1,2,4,12,40,120,288,1092,3072,7776,23600,87120,259200,797160,1847104,
%T A366579 5832000,21523360,63672480,152845056,580921200,1700870400,4368821184,
%U A366579 12550120000,47071589412,130459631616,413939700000,997562438080,3012122557440,11159367815680
%N A366579 a(n) = phi(3^n+1), where phi is Euler's totient function (A000010).
%H A366579 Max Alekseyev, <a href="/A366579/b366579.txt">Table of n, a(n) for n = 0..691</a>
%F A366579 a(n) = phi(3^n+1) = A000010(A034472(n)).
%t A366579 EulerPhi[3^Range[0,30]+1] (* _Paolo Xausa_, Oct 15 2023 *)
%o A366579 (PARI) {a(n) = eulerphi(3^n+1)}
%Y A366579 Cf. A000010, A002592, A034472, A053285, A057935, A057941, A074476, A274909, A295500, A366577, A366578, A366580.
%K A366579 nonn
%O A366579 0,2
%A A366579 _Sean A. Irvine_, Oct 13 2023