# Chain Rule

If I have two functions, f(x,y,z) and g(x,y,z), do I use the chain rule to calculate df/dg?

e.g. df/dg=df/dx df/dy df/dz

Cheers!

Romeo

Are you really trying to find the derivative of one function with respect to the other function?

Are you really trying to find the derivative of one function with respect to the other function?

Is that not possible?

If f is not a function g, then you derivative equals 0, if I'm not mistaken.

Is that not possible?

If I remember correctly, it is possible, but somewhat difficult.

But if f is a function for (x,y,z) and g is a function of (x,y,z) then surely that's possible. Probably I'm wrong if I have to ask about it.

But if f is a function for (x,y,z) and g is a function of (x,y,z) then surely that's possible. Probably I'm wrong if I have to ask about it.

Is there a way that you can justify this?

I'm just wondering if you have more than your intuition.

But if f is a function for (x,y,z) and g is a function of (x,y,z) then surely that's possible. Probably I'm wrong if I have to ask about it.

Hey, ex and x+1 are both function of x. What is the derivative of ex with respect to x+1?

Is there a way that you can justify this?

I'm just wondering if you have more than your intuition.

You're right - just intuition. Which is probably failing me.

You're right - just intuition. Which is probably failing me.

What is your mathematical background? Are you learning (or teaching yourself) calculus right now?

Are you guys ignoring my posts on purpose :( ?

Are you guys ignoring my posts on purpose :( ?

No, but I think that if you're trying to make a point, it is best done explicitly.

Didn't I make my point explicitly? I'm not trying to sound like a douche or anything, I actually thought the same thing as romeo6 when I first red your question.

Didn't I make my point explicitly? I'm not trying to sound like a douche or anything, I actually thought the same thing as romeo6 when I first red your question.

You answered a question with a question. (Granted that is what I did too.)

Did you take a look at my first post in this thread?

Did you take a look at my first post in this thread?

I did. I just missed when I was reviewing the thread. Sorry.

I did. I just missed when I was reviewing the thread. Sorry.

Chill, np. This threat got me thinking. Sometimes some concepts become so instinctive that thinking about them makes knots in your brain.

If f is not a function g, then you derivative equals 0, if I'm not mistaken.

Upon further thought, this is not true.

Since f(x) and g(x) are both functions of x f(x) is, in some sense, dependent on g(x), because g(x)=f-1(x)). The thing that has to be kept in mind, is that, since nothing is specified about f or g, f-1 or g-1 may not themselves be functions, which means that their domains must be restricted in order for them to be differentiable.

However, with the proper assumptions made about f and/or g, the chain rule and the inverse function theorem yield:

$\frac{d(f(g^{-1}(x)))}{dx}=\frac{df}{dx}\frac{1}{\frac{dg}{dx}}$

Mark44
Mentor
If I have two functions, f(x,y,z) and g(x,y,z), do I use the chain rule to calculate df/dg?

e.g. df/dg=df/dx df/dy df/dz
This really doesn't make much sense. You don't calculate the derivative of a function with respect to some other function, but you do calculate the derivative of a function with respect to one of its variables. Here g is a function, not a variable, so df/dg is nonsensical.

For another thing, both functions here have multiple variables, so instead of df/dx, df/dy, and df/dz, you would be working with partial derivatives,
$$\frac{\partial f}{\partial x}, \frac{\partial f}{\partial y}, \text{and} \frac{\partial f}{\partial z}$$

Other notation for these partials is fx, fy, and fz.

This really doesn't make much sense. You don't calculate the derivative of a function with respect to some other function, but you do calculate the derivative of a function with respect to one of its variables. Here g is a function, not a variable, so df/dg is nonsensical.

Except that is exactly what you do when you perform a variable transform.

What do you mean by variable transform?

What Mark44 said makes sense. If you want to derive something with respect to a function you need a functional.

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Mark44
Mentor
I think what you're talking about is a matrix of partial derivatives, which is called the Jacobian. See Jacobian. If so, that's not the same as df/dg as in the original post. The matrix is made up of the partials of each variable in the first system with respect to each variable in the second system, so we're talking about a bunch of partials of variables, with respect to other variables.

Why does it not makes sense to define a derivative of a function with respect to another function?

chiro