positive definiteness of the Stress Polarization Tensor

Can anyone show me  the
 positive definiteness of the Stress Polarization Tensor  P_ijkl, i.e.,

τ_ij^* P_ijkl τ_kl^*≥o,

and further, 

τ_ij^* M_ijkl τ_kl^*≥τ_ij^* P_ijkl τ_kl^*,

where, 
M_ijkl is the elastic compliance tensor,  τ_ij^* is arbitrary stress polarization with τ_ij^*=−L_ijkl  ε_kl^*.

Many many thanks.