# General Relativity/Coordinate systems and the comma derivative

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In General Relativity we write our (4-dimensional) coordinates as . The flat Minkowski spacetime coordinates ("Local Lorentz frame") are , , , and , where is the speed of light, is time, and , , and are the usual 3-dimensional Cartesian space coordinates.

A **comma derivative** is just a convenient notation for a partial derivative with respect to one of the coordinates. Here are some examples:

1.

2.

3.

4.

If several indices appear after the comma, they are all taken to be part of the differentiation. Here are some examples:

1.

2.

Now, we change coordinate systems via the **Jacobian** . The transformation rule is .

Finally, we present the following important theorem:

**Theorem:**

**Proof:** , which by the chain rule is , which is of course .