# The Lie derivative of forms and tensors

The Lie derivative $${L_{v}}$$ can be applied to a $${k}$$-form $${\varphi}$$ by using the pullback of $${\varphi}$$ by the diffeomorphism $${\Phi}$$ associated with the flow of $${v}$$, i.e. applied to $${k}$$ vectors $${w_{I}}$$ we define

$$\displaystyle L_{v}\varphi\left(w_{I}\right)\equiv\underset{\varepsilon\rightarrow0}{\textrm{lim}}\frac{1}{\varepsilon}\left[\varphi\left(\mathrm{d}\Phi_{\varepsilon}\left(w_{I}\right)\right)-\varphi\left(w_{I}\right)\right].$$

$${L_{v}\varphi}$$ thus measures the change in $${\varphi}$$ as its arguments are transported by the local flow of $${v}$$. In the case of a 0-form $${f}$$, this is just the differential or directional derivative $${L_{v}f=v(f)=\mathrm{d}f(v)}$$.

The above illustrates the Lie derivative for a 1-form $${\varphi}$$ with $${\varepsilon=1}$$. $${L_{v}\varphi}$$ is “the difference between $${\varphi}$$ applied to $${w}$$ and $${\varphi}$$ applied to $${w}$$ transported by the local flow of $${v}$$,” so above we have $${L_{v}\varphi(w)=2-1=1}$$ (valid in the limit $${\varepsilon\rightarrow0}$$ if $${\varphi}$$ changes linearly in the range shown).

By using the above definitions of the Lie derivative applied to vectors and 1-forms, and noting that we can derive a Leibniz rule over contraction $${L_{v}(\varphi\left(w\right))=\left(L_{v}\varphi\right)(w)+\varphi\left(L_{v}w\right)}$$, we arrive at an expression for the Lie derivative applied to general tensors, viewed as real-valued mappings on vectors and 1-forms:

\begin{aligned}L_{v}T(\varphi_{1},\ldots,\varphi_{m},w_{1},\ldots,w_{n}) & =v\left(T(\varphi_{1},\ldots,\varphi_{m},w_{1},\ldots,w_{n})\right)\\
& -\sum_{j=1}^{m}T\left(\varphi_{1},\ldots,L_{v}\varphi_{j},\ldots,\varphi_{m},w_{1},\ldots,w_{n}\right)\\
& -\sum_{j=1}^{n}T\left(\varphi_{1},\ldots,\varphi_{m},w_{1},\ldots,L_{v}w_{j},\ldots,w_{n}\right)
\end{aligned}

In a holonomic frame, this yields an expression for the Lie derivative of a tensor in terms of coordinates

\begin{aligned}L_{v}T^{\mu_{1}\dots\mu_{m}}{}_{\sigma_{1}\dots\sigma_{n}} & =v^{\lambda}\frac{\partial}{\partial x^{\lambda}}T^{\mu_{1}\dots\mu_{m}}{}_{\sigma_{1}\dots\sigma_{n}}\\
& -\sum_{j=1}^{m}\left(\frac{\partial v^{\mu_{j}}}{\partial x^{\lambda}}\right)T^{\mu_{1}\dots\mu_{j-1}\lambda\mu_{j+1}\dots\mu_{m}}{}_{\sigma_{1}\dots\sigma_{n}}\\
& +\sum_{j=1}^{n}\left(\frac{\partial v^{\lambda}}{\partial x^{\sigma_{j}}}\right)T^{\mu_{1}\dots\mu_{m}}{}_{\sigma_{1}\dots\sigma_{j-1}\lambda\sigma_{j+1}\dots\sigma_{n}}.
\end{aligned}

From this we can confirm that the Lie derivative satisfies the Leibniz rule over the tensor product, and therefore is a derivation of degree 0 on both the tensor algebra and the exterior algebra.