# Metric components transformation under change of coordinates

I have been studying Lie derivatives and some applications. While searching the web I found a refence with the following statement: For a general Riemannian manifold $M$, take a tangent vector field $k=k^\mu \partial _\mu$ and consider the infinitesimal coordinate transformation, $$x^\mu \to x^\mu + \alpha k^\mu~,$$ where $|\alpha|<<1$. Then it is possible to find that the metric components $g_{\mu \nu}$ transform as $$g_{\mu \nu} \to g_{\mu \nu} + \alpha(\partial_\mu k_\nu + \partial _\nu k_\mu + k^\sigma \partial _\sigma g_{\mu\nu} ) +O(\alpha^2)~.$$

Now, how does the author get this? Does the author just uses the usual transformation law for a (0,2) tensor? This does not seem to be the case since he finds so many terms, moreover I do not know the inverse transformation so I would't be able to apply it. Can somebody help me?

The metric being a rank $(0,2)$ tensor transforms under general coordinate transformations $x^\mu \to x'^\mu(x)$ as $$g'_{\mu\nu} (x') = \frac{ \partial x^\rho}{ \partial x'^\mu } \frac{ \partial x^\sigma }{ \partial x'^\nu } g_{\rho\sigma} (x)$$

Now set $x'^\mu (x) = x^\mu + \alpha k^\mu(x)$ in the above expression and take a limit of small $\alpha$. You should then get $$g'_{\mu\nu}(x) = g_{\mu\nu}(x) - \alpha ( g_{\mu\rho} \partial_\nu k^\rho + g_{\nu\rho} \partial_\mu k^\rho + k^\rho \partial_\rho g_{\mu\nu} ) + {\cal O} (\alpha^2)$$

The above is the correct version of the transformation. I believe you have gotten it wrong in the question (v1)

• This must be a really stupid question but what do you mean substitute in the expression?Make a Taylor expansion?
– PML
Jan 29 '16 at 16:17
• BTW, to be clear, the inverse transformation is $x^\mu = x'^\mu - a k^\mu + O(a^2)$. Jan 29 '16 at 16:20
• @PML - I mean, substitute the expression for $x'^\mu$ that I've written down into the first equation and then do a Taylor expansion in $\alpha$. Jan 29 '16 at 16:23
• @Prahar why do you have $\partial_\rho g_{\mu\nu}$ instead of $\partial_\rho g_{\mu\nu}'$ when you Taylor expand? Feb 6 '20 at 16:51
• I write $\partial_\rho g'_{\mu\nu} = \partial_\rho g_{\mu\nu} + {\cal O}( \alpha)$. Feb 6 '20 at 16:52