Akm[k_,m_]:=Piecewise[{{0, k != 3 || (m != -3 && m != -2 && m != -1 && m != 0 && m != 1 && m != 2 && m != 3)}, {-A[3, 3], k == 3 && m == -3}, {A[3, 2], k == 3 && (m == -2 || m == 2)}, {-A[3, 1], k == 3 && m == -1}, {A[3, 0], k == 3 && m == 0}, {A[3, 1], k == 3 && m == 1}}, A[3, 3]]