Akm[k_,m_]:=Piecewise[{{0, (k != 4 && (k != 2 || m != 0)) || (m != -4 && m != 0 && m != 4)}, {(5*(Sqrt[21]*Mb - Sqrt[21]*Me1 - Sqrt[35]*Me2))/21, k == 2 && m == 0}, {(3*(5*Sqrt[30]*Me1 - 15*Sqrt[2]*Me2 - (8*I)*Sqrt[30]*Me3))/40, k == 4 && m == -4}, {(3*(4*Sqrt[21]*Mb + 3*Sqrt[21]*Me1 + 3*Sqrt[35]*Me2))/28, k == 4 && m == 0}}, (3*(5*Sqrt[30]*Me1 - 15*Sqrt[2]*Me2 + (8*I)*Sqrt[30]*Me3))/40]