Akm[k_,m_]:=Piecewise[{{(Ea2u + 3*(Et1u + Et2u))/7, k == 0 && m == 0}, {I*Sqrt[5/14]*(2*Ea2u - 3*Et1u + Et2u), k == 4 && (m == -3 || m == 3)}, {(2*Ea2u - 3*Et1u + Et2u)/2, k == 4 && m == 0}, {(-13*Sqrt[11/21]*(4*Ea2u + 5*Et1u - 9*Et2u))/60, k == 6 && (m == -6 || m == 6)}, {(((-13*I)/6)*(4*Ea2u + 5*Et1u - 9*Et2u))/Sqrt[210], k == 6 && (m == -3 || m == 3)}, {(26*(4*Ea2u + 5*Et1u - 9*Et2u))/105, k == 6 && m == 0}}, 0]