Test for dependence/independence of multiple variables

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Discussion Overview

The discussion revolves around the dependence and independence of multiple variables in the context of multi-variable calculus. Participants explore how to determine the relationships between variables, particularly when dealing with functions of several variables and their derivatives.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about determining which variables depend on others when there are four or more variables involved, using the example of w(r,s), r(u,v), and s(u,v).
  • Another participant questions the dependence of v, noting that the original post does not clarify what v depends on.
  • A participant seeks to understand if it is possible to know if v depends on other variables based solely on the given functions.
  • It is suggested that u and v are independent variables, while r and s are dependent on u and v, and that w is ultimately dependent on u and v as well.
  • A follow-up question is raised regarding the relationship between a variable being independent or dependent and whether its derivative is zero.
  • One participant clarifies that a variable being independent or dependent does not directly relate to its derivative being zero, emphasizing the need for partial derivatives in multi-variable functions.
  • There is a discussion about the implications of independence on the derivatives, with a suggestion that if u and v are independent, then dv/du could be zero.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the dependence of v or the implications of independence on derivatives. Multiple viewpoints and uncertainties remain regarding the relationships between the variables and their derivatives.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the relationships between the variables and the definitions of independence and dependence. The discussion does not resolve these complexities.

mishima
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I don't think this is related to linear dependence/independence of linear algebra. I am talking about multi-variable calculus.

I am getting confused with being able to quickly tell which variables depend on others when there are 4+ variables involved. For example, suppose I have w(r,s), r(u,v), and s(u,v). Then I am asked to find dv/du. If v is dependent on u, this is non-zero...but how can I tell?

If I rewrite v in terms of r and u to get v(r,u) then it seems clear v is dependent on u, but I am uncertain.
 
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What is v? On what does it depend? You just tell us what w,r,s depend on, but not what v depends on.
 
That's what I'm trying to figure out/understand. Are you saying just given w(r,s), r(u,v), and s(u,v) that there is no way to know if v depends on one of the other variables?
 
mishima said:
Are you saying just given w(r,s), r(u,v), and s(u,v) that there is no way to know if v depends on one of the other variables?
From the given information, with r and s being functions of u and v, u and v would be the independent variables, with r and s being dependent on u and v.

The function w has r and s as independent variables, and w being dependent. But since both r and s depend on u and v, then ulitimately w is a function of u and v, so is dependent on those two variables. Making a tree diagram is helpful in these situations.
 
That makes sense, maybe a followup question to focus in on my confusion...

Does a variable being independent or dependent have anything to do with whether or not its derivative is zero? In other words, now that we have established that u and v are IV here, is dv/du necessarily 0? Thanks.
 
mishima said:
Does a variable being independent or dependent have anything to do with whether or not its derivative is zero?
No, but see below. If we're talking about single-variable functions, such as y = f(t), if dy/dt = 0, all this means is that y is constant. For the functions you show in post #1, you have to use partial derivatives, as in the partial derivative of r with respect to one of its independent variables.

mishima said:
In other words, now that we have established that u and v are IV here, is dv/du necessarily 0?
That's sort of a different question. If u and v are unrelated (as in being independent) the du/dv = 0 and dv/du = 0.
 

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