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【SEOtext】0. General Formulation, [0] Action Principle, (1) Action, An action is defined as such a functional I that the classical equation of motion is derived from vanishing of variation of I. δI(q)/δqi(t)=0, δI(φ)/δφα(x)=0 (2) Symmetry, If I is invariant under an operator U, that is: I(U(φ))=I(φ), then the equation of motion is invariant under U. δI(φ)/δφα(x)=0⇔δI(U(φ))/δ[U(φ)]α(x)=0