Akm[k_,m_]:=Piecewise[{{(2*Eeg + 3*Et2g)/5, k == 0 && m == 0}, {(-I)*Sqrt[14/5]*(Eeg - Et2g), k == 4 && (m == -3 || m == 3)}, {(-7*(Eeg - Et2g))/5, k == 4 && m == 0}}, 0]