Akm[k_,m_]:=Piecewise[{{A[0, 0], k == 0 && m == 0}, {-(Sqrt[10/7]*A[4, 0]), k == 4 && m == -3}, {A[4, 0], k == 4 && m == 0}, {Sqrt[10/7]*A[4, 0], k == 4 && m == 3}, {(Sqrt[77/3]*A[6, 0])/8, k == 6 && (m == -6 || m == 6)}, {(Sqrt[35/6]*A[6, 0])/4, k == 6 && m == -3}, {A[6, 0], k == 6 && m == 0}, {-(Sqrt[35/6]*A[6, 0])/4, k == 6 && m == 3}}, 0]