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 A366669 #11 Jan 20 2024 12:34:33
%S A366669 1,10,100,720,9792,90900,990000,9090900,94117632,681410880,9897840000,
%T A366669 86925373920,979102080000,9080325951840,95255567232000,
%U A366669 712493107200000,9926748531589120,90004044661864320,989999010000000000,9090909090909090900,97910150554895155200
%N A366669 a(n) = phi(10^n+1), where phi is Euler's totient function (A000010).
%H A366669 Max Alekseyev, <a href="/A366669/b366669.txt">Table of n, a(n) for n = 0..331</a>
%F A366669 a(n) = A000010(A062397(n)). - _Paul F. Marrero Romero_, Nov 10 2023
%t A366669 EulerPhi[10^Range[0,20] + 1] (* _Paul F. Marrero Romero_, Nov 10 2023 *)
%o A366669 (PARI) {a(n) = eulerphi(10^n+1)}
%Y A366669 Cf. A000010, A003021, A057934, A062397, A119704, A295503, A344897, A366668.
%Y A366669 Cf. A053285, A366579, A366608, A366618, A366630, A366639, A366658, A366667, A366690, A366716.
%K A366669 nonn
%O A366669 0,2
%A A366669 _Sean A. Irvine_, Oct 15 2023