Akm[k_,m_]:=Piecewise[{{(2*Eeg)/5 + (3*Et2g)/5, k == 0 && m == 0}, {(3*Sqrt[7/10]*(Eeg - Et2g))/2, k == 4 && (m == -4 || m == 4)}, {(21*(Eeg - Et2g))/10, k == 4 && m == 0}}, 0]