Can Cross Derivatives be Equal in Partial Derivatives?

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SUMMARY

The discussion centers on the equality of cross derivatives in the context of partial derivatives, specifically examining the function x(i,j) with a=dx/di and b=dx/dj. The participants question the validity of substituting derivatives and taking the derivative of one variable with respect to another, which is generally not permissible when i and j are independent variables. The confusion arises from the notation and the theoretical implications of cross derivatives, which was ultimately resolved by one participant using a Lagrangian approach.

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chicago77
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So this is a theoretical question I am not sure about with partial derivatives.

Say we have function x(i,j) with a=dx/di and b=dx/dj

Now is this logical when the cross derivatives are required to be equal?

db/da --> substitute b
= d^2x/(dadj) substitute da
= d^2x/((d^2x/di)dj)
=di/dj

Is this allowed?
 
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chicago77 said:
So this is a theoretical question I am not sure about with partial derivatives.

Say we have function x(i,j) with a=dx/di and b=dx/dj

Now is this logical when the cross derivatives are required to be equal?

db/da --> substitute b
Right off the bat I'm not following this. This says you are taking the (partial) derivative with respect to dx/dj, which makes not sense at all.

A small part of my difficulty is with your notation - dx/di - which is supposed to be the partial derivative of x with respect to i.
chicago77 said:
= d^2x/(dadj) substitute da
= d^2x/((d^2x/di)dj)
=di/dj

Is this allowed?
Presumably i and j (which are really bad choices for the names of variables) are independent variables, so you wouldn't ordinarily take the derivative (partial or otherwise) of one with respect to the other.
 
nvm I have solved this with a lagrangian.
 
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