Akm[k_,m_]:=Piecewise[{{0, k != 4 || (m != -3 && m != 0 && m != 3)}, {(-1/2 + I/2)*Sqrt[15]*Mt1u, k == 4 && m == -3}, {(Sqrt[21]*Mt1u)/2, k == 4 && m == 0}}, (1/2 + I/2)*Sqrt[15]*Mt1u]