Mertens function

(*Mathematica Mats Granvik 7.4.2014*)
Clear[A, t, n, k, a, nn];
nn = 11;
a = Table[StringJoin[{"x", ToString[n - 1]}], {n, 1, nn}]
Length[a]
t[n_, 1] = If[n <= 3, 1, a[[n]]];
t[n_, k_] :=
  t[n, k] =
   If[n >= k,
    If[n <= 3, 1,
     If[And[k > 1],
      If[Or[k == 2, k == 3],
       t[n, k - 1] - Sum[t[n - i, k], {i, 1, k - 1}],
       If[k >= 4,
        Sum[t[n - i, k - 1], {i, 1, k - 2}] -
         Sum[t[n - i, k], {i, 1, k - 1}], 0], 0], 0, 0], 0], 0];
A = Table[Table[t[n, k], {k, 1, nn}], {n, 1, nn}];
MatrixForm[A]
Inverse[A][[2 ;; nn, 1]]