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The nonlinear force is the electrodynamic part of the gradient of the momentum-flux-density of a more general form than used by Landau and Lifshitz (Ref. 5). The generalization with respect to dispersion was confirmed by consistency with the derivation from other models, e.g., the two-fluid model (Ref. 6). Because Landau and Lifshitz (Ref. 5) used the expression "ponderomotive force" for gas dynamic (thermokinetic) and other forces, the expression "nonlinear force" for its electrodynamic part may be preferred with respect to its quadratic terms. Shearer and Eddleman (Ref. 7) specialized the general derivation to reproduce the electric term (without the magnetic term) of the nonlinear force, as used in microwave theory (Ref. 8), from which I needed further specializations to derive the general expression (Ref. 6).

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