Akm[k_,m_]:=Piecewise[{{(2*Ee)/5 + (3*Et2)/5, k == 0 && m == 0}, {(3*Sqrt[7/10]*(Ee - Et2))/2, k == 4 && (m == -4 || m == 4)}, {(21*(Ee - Et2))/10, k == 4 && m == 0}}, 0]