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a(n) = phi(6^n-1)/n, where phi is Euler's totient function (A000010).
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%I #27 Jun 17 2024 15:46:28

%S 4,12,56,216,1240,5040,31992,139968,828576,3720000,25238048,104509440,

%T 803499840,3687014016,24373440000,110630707200,790546192128,

%U 3463116249600,25522921047520,108957312000000,816244048599840,3924124012353600,26682733370563200

%N a(n) = phi(6^n-1)/n, where phi is Euler's totient function (A000010).

%H Seiichi Manyama, <a href="/A295496/b295496.txt">Table of n, a(n) for n = 1..100</a>

%H Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TotientFunction.html">Totient Function</a>.

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Euler%27s_totient_function">Euler's totient function</a>.

%t Array[EulerPhi[6^# - 1]/# &, 25] (* _Paolo Xausa_, Jun 17 2024 *)

%o (PARI) {a(n) = eulerphi(6^n-1)/n}

%Y Column k=6 of A369291.

%Y phi(k^n-1)/n: A011260 (k=2), A027385 (k=3), A027695 (k=4), A027741 (k=5), this sequence (k=6), A027743 (k=7), A027744 (k=8), A027745 (k=9), A295497 (k=10), A319166 (k=11).

%K nonn

%O 1,1

%A _Seiichi Manyama_, Nov 22 2017