Akm[k_,m_]:=Piecewise[{{(Eb1u + Eb2u + Eb3u)/3, k == 0 && m == 0}, {(-5*(Eb2u - Eb3u))/(2*Sqrt[6]), k == 2 && (m == -2 || m == 2)}, {(5*(2*Eb1u - Eb2u - Eb3u))/6, k == 2 && m == 0}}, 0]