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%I #11 Jan 04 2024 19:07:20
%S 2,16,108,640,5600,36288,264992,1536000,12387168,85120000,658519752,
%T 3135283200,32296336800,216063877120,1450340640000,8333819904000,
%U 77537969371008,488237947481088,3790563394976072,19162214400000000,170264753751665664,1245495178700551680
%N a(n) = phi(7^n-1), where phi is Euler's totient function (A000010).
%H Max Alekseyev, <a href="/A366635/b366635.txt">Table of n, a(n) for n = 1..388</a>
%t EulerPhi[7^Range[30] - 1] (* _Wesley Ivan Hurt_, Oct 15 2023 *)
%o (PARI) {a(n) = eulerphi(7^n-1)}
%Y phi(k^n-1): A053287 (k=2), A295500 (k=3), A295501 (k=4), A295502 (k=5), A366623 (k=6), this sequence (k=7), A366654 (k=8), A366663 (k=9), A295503 (k=10), A366685 (k=11), A366711 (k=12).
%Y Cf. A000010, A024075, A366632, A366633, A366634.
%K nonn
%O 1,1
%A _Sean A. Irvine_, Oct 15 2023