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%I #30 Jan 31 2024 20:50:21
%S 1,4,32,280,5040,37856,829440,15676416,589032000,10374307328,
%T 388566097920,7619466454080,390751784579520,11138729990400000,
%U 575561351791902720,24328359845627701248,1640651748984970444800,34709116765970413844280,2459108342476800000000000
%N a(n) = phi(n^n - 1)/n where phi is A000010.
%C Main diagonal of the array T(n,k) = phi(n^k-1)/k for n > 1 and k > 1, which starts
%C 1, 2, 2, 6, 6, 18, 16, ... A011260
%C 2, 4, 8, 22, 48, 156, 320, ... A027385
%C 4, 12, 32, 120, 288, 1512, 4096, ... A027695
%C 4, 20, 48, 280, 720, 5580, 14976, ... A027741
%C 12, 56, 216, 1240, 5040, 31992, 139968, ... A295496
%C 8, 36, 160, 1120, 6048, 37856, 192000, ... A027743
%C 18, 144, 432, 5400, 23328, 254016, 829440, ... A027744
%H Seiichi Manyama, <a href="/A319183/b319183.txt">Table of n, a(n) for n = 2..50</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 Table[EulerPhi[n^n-1]/n,{n,20}] (* _Harvey P. Dale_, Aug 04 2020 *)
%o (PARI) {a(n) = eulerphi(n^n-1)/n}
%Y A diagonal of A369291.
%Y Cf. A000010, A006486, A027385.
%K nonn
%O 2,2
%A _Seiichi Manyama_, Sep 12 2018