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A classical field-theory soliton is a smooth, localized finite-energy solution that behaves as a persistent object. Stability may follow from a topological charge or from a balance among energy terms with different scaling behavior.

A classical field-theory soliton is a smooth, spatially localized, finite-energy solution which retains its identity under time evolution and is stable against small perturbations, commonly because of a topological charge or a balance between energy terms with different scaling behavior.

For a static field writewhereUnder the Derrick scaling ${\varphi }_{\lambda }(\mathbf{x})=\varphi (\lambda \mathbf{x})$, a change of variables givesA static solution must be stationary under this variation, so the Derrick virial identity isAll three energies are nonnegative. For $D\ge 4$, every coefficient is nonpositive and the coefficient of the strictly positive $E_2$ of any nonconstant field is negative. The identity is impossible. Thus, when the quartic-gradient term is available,For $D=2$ or $3$, the $E_4$ term has the opposite sign to at least one other term and the Derrick theorem does not rule out a soliton. If $\kappa =0$ from the outset, the identity reduces to $(2-D)E_2-D{E}_0=0$, recovering the stronger standard obstruction for $D\ge 2$.