%I #18 Nov 01 2017 12:23:38
%S 21,351,7826,210456,6047412,181410426,5597460306,176319684486,
%T 5642220380006,182807931073356,5982804736593846,197432556035646576,
%U 6560835699716880876,219319364810710999926,7369130657373455467336,248708159686089506901366
%N Number of distinct n X 2 toroidal 0..5 arrays.
%H Andrew Howroyd, <a href="/A184289/b184289.txt">Table of n, a(n) for n = 1..100</a>
%t T[n_, k_] := (1/(n*k))*Sum[Sum[EulerPhi[c]*EulerPhi[d]*6^(n*(k/LCM[c, d])), {d, Divisors[k]}], {c, Divisors[n]}]; a[n_] := T[n, 2]; Array[a, 16] (* _Jean-François Alcover_, Nov 01 2017, after _Andrew Howroyd_ *)
%o (PARI) a(n) = my(k=2); (1/(n*k)) * sumdiv(n, c, sumdiv(k, d, eulerphi(c) * eulerphi(d) * 6^(n*k/lcm(c, d)))); \\ _Michel Marcus_, Nov 01 2017
%Y Column 2 of A184291.
%K nonn
%O 1,1
%A _R. H. Hardin_, Jan 10 2011
%E Terms a(10) and beyond from _Andrew Howroyd_, Sep 27 2017