Akm[k_,m_]:=Piecewise[{{(4*Egu + 3*Et2u)/7, k == 0 && m == 0}, {(39*Sqrt[33/35]*(Egu - Et2u))/32, k == 6 && (m == -6 || m == 6)}, {(-429*(Egu - Et2u))/(80*Sqrt[14]), k == 6 && (m == -4 || m == 4)}, {(-429*Sqrt[3/7]*(Egu - Et2u))/160, k == 6 && (m == -2 || m == 2)}, {(429*(Egu - Et2u))/560, k == 6 && m == 0}}, 0]