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 A366639 #12 Jan 20 2024 12:26:10
%S A366639 1,4,20,168,1200,7600,43200,407680,2712832,19707408,112560000,
%T A366639 945677920,6768230400,47530457728,271289229120,2096760960000,
%U A366639 16569393144832,116315256993600,597938524646400,5699431359135360,38890647857280000,270061302781670400
%N A366639 a(n) = phi(7^n+1), where phi is Euler's totient function (A000010).
%H A366639 Max Alekseyev, <a href="/A366639/b366639.txt">Table of n, a(n) for n = 0..387</a>
%F A366639 a(n) = A000010(A034491(n)). - _Paul F. Marrero Romero_, Nov 06 2023
%t A366639 EulerPhi[7^Range[0,21] + 1] (* _Paul F. Marrero Romero_, Nov 05 2023 *)
%o A366639 (PARI) {a(n) = eulerphi(7^n+1)}
%Y A366639 Cf. A034491, A000010, A057937, A227575, A366635, A366636, A366637, A366638.
%Y A366639 Cf. A053285, A366579, A366608, A366618, A366630, A366658, A366667, A366669, A366690, A366716.
%K A366639 nonn
%O A366639 0,2
%A A366639 _Sean A. Irvine_, Oct 15 2023