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Ivancevic_Applied-Diff-Geom

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<strong>Applied</strong> Manifold <strong>Geom</strong>etry 193The introduction of df leads to the following definition of the Lie derivative.The directional or Lie derivative L X : C k (M, R) → C k−1 (M, R) of afunction f ∈ C k (M, R) along a vector–field X is defined byL X f(m) = X[f](m) = df(m) · X(m),for any m ∈ M. Denote by X[f] = df(X) the map M ∋ m ↦→ X[f](m) ∈ R.If f is F −valued, the same definition is used, but now X[f] is F −valued.If a local chart (U, φ) on an n−manifold M has local coordinates(x 1 , ..., x n ), the local representative of X[f] is given by the functionL X f = X[f] = X i ∂ x if.Evidently if f is C k and X is C k−1 then X[f] is C k−1 .Let ϕ : M → N be a diffeomorphism. Then L X is natural with respectto push–forward by ϕ. That is, for each f ∈ C k (M, R),L ϕ∗ X(ϕ ∗ f) = ϕ ∗ L X f,i.e., the following diagram commutes:C k ϕ(M, R) ∗ ✲ C k (N, R)L X❄❄C k (M, R) ✲ C k (N, R)ϕ ∗L ϕ∗ XAlso, L X is natural with respect to restrictions. That is, for U open inM and f ∈ C k (M, R),L X|U (f|U) = (L X f)|U,where |U : C k (M, R) → C k (U, R) denotes restriction to U, i.e., the followingdiagram commutes:C k |U(M, R) ✲ C k (U, R)L XL X|U❄❄C k (M, R) ✲ C k (U, R)|USince ϕ ∗ = (ϕ −1 ) ∗ the Lie derivative is also natural with respect topull–back by ϕ. This has a generalization to ϕ−related vector–fields as

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