A184287 - OEIS

%I #17 Nov 01 2017 12:23:31

%S 165,49075,20346485,9536816875,4768372070757,2483526896194175,

%T 1330460821097243445,727595761432647851875,404219867454740749892485,

%U 227373675443239212037148635,129189588320018269798974609525,74014868308343773086872161914375

%N Number of distinct n X 4 toroidal 0..4 arrays.

%H Andrew Howroyd, <a href="/A184287/b184287.txt">Table of n, a(n) for n = 1..100</a>

%t T[n_, k_] := (1/(n*k))*Sum[Sum[EulerPhi[c]*EulerPhi[d]*5^(n*k/LCM[c, d]), {d, Divisors[k]}], {c, Divisors[n]}]; a[n_] := T[n, 4]; Array[a, 12] (* _Jean-François Alcover_, Nov 01 2017, after _Andrew Howroyd_ *)

%o (PARI) a(n) = my(k=4); (1/(n*k)) * sumdiv(n, c, sumdiv(k, d, eulerphi(c) * eulerphi(d) * 5^(n*k/lcm(c, d)))); \\ _Michel Marcus_, Nov 01 2017

%Y Column 4 of A184288.

%K nonn

%O 1,1

%A _R. H. Hardin_, Jan 10 2011

%E Terms a(6) and beyond from _Andrew Howroyd_, Sep 27 2017