Akm[k_,m_]:=Piecewise[{{(Eau + 3*(Etu1 + Etu2))/7, k == 0 && m == 0}, {(-3*Sqrt[5/14]*(2*Eau - 3*Etu1 + Etu2))/4, k == 4 && (m == -4 || m == 4)}, {(-3*(2*Eau - 3*Etu1 + Etu2))/4, k == 4 && m == 0}, {(-39*Sqrt[11/35]*Mtu)/8, k == 6 && (m == -6 || m == 6)}, {(-39*(4*Eau + 5*Etu1 - 9*Etu2))/(40*Sqrt[14]), k == 6 && (m == -4 || m == 4)}, {(429*Mtu)/(40*Sqrt[7]), k == 6 && (m == -2 || m == 2)}, {(39*(4*Eau + 5*Etu1 - 9*Etu2))/280, k == 6 && m == 0}}, 0]