Part 1 - partial derivatives

The series' table of contents

As I mentioned in the introduction, I assume that the reader knows what a derivative of a function is. It is a good foundation, but to get our hands wet in relativity, we need to expand that concept a bit. Let's then get to know the partial derivative. What is it?

Let's remember the ordinary derivatives first. We denote a derivative of a function f(x) as f'(x) or \frac{df}{dx}. It means, basically, how fast the value of the function changes while we change the argument x. For example, when f(x) = x^2, then \frac{df}{dx} = 2x.

But what if the function depends on more than one variable? Like if we have a function f(x,y) = x^2 + y^2 that assigns to each point of the plane the square of its distance from the origin. How do we even define the derivative of such a function?