Intrinsic permutation symmetry of nonlinear susceptibility
= Intrinsic permutation symmetry of nonlinear susceptibility
Commuting input field factors permit the response kernel to be symmetrized under interchange of input index-frequency pairs. For example $\chi^{(2)}_{ijk}(\omega_s;\omega_1,\omega_2)=\chi^{(2)}_{ikj}(\omega_s;\omega_2,\omega_1)$. This does not authorize exchanging indices while keeping unequal <frequency> arguments in their old slots, nor exchanging the output index without further assumptions.