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