= Kleinman symmetry
{c}
Far from relevant resonances, when <frequency> dispersion of the nonlinear response can be neglected, a susceptibility can acquire full permutation symmetry among spatial indices. For second order this reduces a general symmetric-input <tensor>'s 18 components to 10 before material point-group reductions. Mere transparency does not establish this approximation. The name refers to <D. A. Kleinman>.<Nonlinear Dielectric Polarization in Optical Media by D. A. Kleinman>[abstract, electronic high-frequency response and additional symmetry constraints]{ref}
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