= Isotropic fourth-rank susceptibility
{title2=$\chi_{ijkl}=a\delta_{ij}\delta_{kl}+b\delta_{ik}\delta_{jl}+c\delta_{il}\delta_{jk}$}
An isotropic fourth-rank polar <tensor> has the displayed form. If its three input slots are fully interchangeable at the same relevant <frequency> arguments, then $a=b=c$, giving $\chi_{xxxx}=3\chi_{xxyy}$. For mixed-sign or unequal <frequencies>, intrinsic permutation exchanges the arguments too; it need not make all coefficients at one fixed ordering equal.
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