Akm[k_,m_]:=Piecewise[{{(Ea2u + 3*(Et1u + Et2u))/7, k == 0 && m == 0}, {(-3*Sqrt[5/14]*(2*Ea2u - 3*Et1u + Et2u))/4, k == 4 && (m == -4 || m == 4)}, {(-3*(2*Ea2u - 3*Et1u + Et2u))/4, k == 4 && m == 0}, {(-39*(4*Ea2u + 5*Et1u - 9*Et2u))/(40*Sqrt[14]), k == 6 && (m == -4 || m == 4)}, {(39*(4*Ea2u + 5*Et1u - 9*Et2u))/280, k == 6 && m == 0}}, 0]