Akm[k_,m_]:=Piecewise[{{A[0, 0], k == 0 && m == 0}, {A[2, 0], k == 2 && m == 0}, {-A[4, 3], k == 4 && m == -3}, {A[4, 0], k == 4 && m == 0}, {A[4, 3], k == 4 && m == 3}, {A[6, 6], k == 6 && (m == -6 || m == 6)}, {-A[6, 3], k == 6 && m == -3}, {A[6, 0], k == 6 && m == 0}, {A[6, 3], k == 6 && m == 3}}, 0]