[Math] Is this contraction of metric tensor derivatives symmetric

differential-geometrysymmetry

A couple of times when I've tried to prove symmetries of various tensors (for learning), I've ended up with the expression below, and the fact that either a) I made mistake, or b) the expression is symmetric with respect to switching k and l.

$$
\frac{\partial g_{ij}}{\partial x^k} \frac{\partial g^{ij}}{\partial x^l}
$$

Where $g_{..}$ and $g^{..}$ are the covariant and contravariant metric tensor respectively, and $x^.$ is the coordinate.

Is the expression symmetric wrt switching $k$ and $l$? If so, is it possible to prove this using only indicial notation?

Best Answer

Since the product rule tells us $0 = \partial( g g^{-1} ) = (\partial g) g^{-1} + g (\partial g^{-1})$, we have a formula for the derivative of the inverse metric:

$$ \partial_l g^{ij} = -g^{ia} g^{jb} \partial_l g_{ab}.$$

Substituting this in to your expression we get

$$ -g^{ia} g^{jb} \partial_l g_{ab} \partial_k g_{ij}.$$

If we swap the dummy indices $a \leftrightarrow i$, $b \leftrightarrow j$ then this is equal to

$$ -g^{ai} g^{jb} \partial_l g_{ij} \partial_k g_{ab};$$

so it's symmetric in $k$ and $l$.

Related Question