Akm[k_,m_]:=Piecewise[{{A[0, 0], k == 0 && m == 0}, {(Sqrt[105/11]*A[6, 0])/2, k == 6 && (m == -6 || m == 6)}, {-(Sqrt[7/2]*A[6, 0]), k == 6 && (m == -4 || m == 4)}, {-(Sqrt[21]*A[6, 0])/2, k == 6 && (m == -2 || m == 2)}, {A[6, 0], k == 6 && m == 0}}, 0]