Notation for 2nd-order mixed derivatives

In summary, there are multiple ways to denote similarity transformations and rotations using quaternions, and there is no one "correct" way to represent the partial derivative \partial/\partial y\left(\partial f/\partial x\right). It is important to pay attention to context and to consult reliable sources when determining the appropriate notation.
  • #1
kingwinner
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Homework Statement


According to Wikipedia and one of my calculus textbooks: (I am used to this notation)
378b7ac113c92212162a0f23c27c1175.png

But I've seen another source writing fxy=∂2f/∂x∂y=∂xyf which is totally inconsistent with the above. How come?

Which one is standard? Is the second one wrong, or is it acceptable? The second one is driving me crazy...is it ever used anywhere?


Homework Equations


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The Attempt at a Solution


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Thank you!
 
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  • #2
If you replace f_xy in your first example with f_yx, then I'm with you. For basically all examples of interest in physics or the sciences, f_xy=f_yx. Hence, the free interchange of x and y. In mathematics there are examples of functions where f_xy is not equal to f_yx.
 
  • #3
Dick said:
If you replace f_xy in your first example with f_yx, then I'm with you. For basically all examples of interest in physics or the sciences, f_xy=f_yx. Hence, the free interchange of x and y. In mathematics there are examples of functions where f_xy is not equal to f_yx.

fxy means (fx)y, no?
So I would interpret fxy as taking partial derivative w.r.t. x first and then w.r.t. y which is consistent with my calculus textbook.

However, in my studies of partial differential equations (which I would classify as math rather than science), they write fxy=∂2f/∂x∂y=∂xyf which is inconsistent with the above.
2f/∂x∂y means taking partial derivative w.r.t. y first and then w.r.t. x, but fxy to me is the opposite.

Which notation is standard in mathematics?

Thanks!
 
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  • #4
You know, I've never thought about it that hard. Wikipedia says ∂2f/∂x∂y is f_yx. I guess I'll trust them. The operator notation is clearer.
 
  • #5
kingwinner said:
Which one is standard? Is the second one wrong, or is it acceptable? The second one is driving me crazy...is it ever used anywhere?
http://mathworld.wolfram.com/MixedPartialDerivative.html, for one.

What is the "correct" way to denote a similarity transformation:
[tex]QAQ^{-1}[/tex] or [tex]Q^{-1}AQ[/tex]

What is the "correct" way to represent a rotation by angle [itex]\theta[/itex] about an axis [itex]\hat u[/itex] as a quaternion:
[tex]\bmatrix \cos \theta/2 \\ \hat u \sin \theta/2\endbmatrix[/tex] or [tex]\bmatrix \cos \theta/2 \\ -\hat u \sin \theta/2\endbmatrix[/tex] or [tex]\bmatrix\hat u \sin \theta/2 \\ \cos \theta/2 \endbmatrix[/tex] or [tex]\bmatrix -\hat u \sin \theta/2 \\ \cos \theta/2 \endbmatrix[/tex]

What is the "correct" way to represent [itex]\partial/\partial y\left(\partial f/\partial x\right)[/itex]:
[tex]f_{xy}[/itex] or [tex]f_{yx}[/tex]The correct answer is: Asking which way is the "correct" way is wrong. There are lots of cases where different nomenclatures are used in different places. I just gave three examples; there are many more. You need to recognize that sometimes there are multiple ways to write things. Sometimes the chosen nomenclature is obvious from context. Sometimes it isn't so obvious; hopefully the author of a paper describes the nomenclature. Sometimes not. Such is life.
 
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What is the notation for 2nd-order mixed derivatives?

The notation for 2nd-order mixed derivatives is d2f(x,y)/dxdy or d2f(x,y)/dydx.

How do you calculate a 2nd-order mixed derivative?

To calculate a 2nd-order mixed derivative, you first take the derivative of the function with respect to one variable, and then take the derivative of that result with respect to the other variable.

What does a 2nd-order mixed derivative represent?

A 2nd-order mixed derivative represents the rate of change of the rate of change of a function with respect to two different variables.

What is the difference between a 2nd-order mixed derivative and a 2nd-order partial derivative?

A 2nd-order mixed derivative involves taking the derivative with respect to two different variables, while a 2nd-order partial derivative involves taking the derivative with respect to one variable twice.

Can you give an example of a physical application of 2nd-order mixed derivatives?

One example of a physical application of 2nd-order mixed derivatives is in the study of heat transfer, where the rate of change of temperature with respect to both time and distance can be represented by a 2nd-order mixed derivative.

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