Are There General Rules for Limits in Multivariable Real-Valued Functions?

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Discussion Overview

The discussion revolves around the generalization of limit rules for multivariable real-valued functions, specifically examining whether the established rules for limits in single-variable calculus can be applied in a similar manner to functions of multiple variables. The scope includes theoretical considerations and potential pathological cases, as well as inquiries about analogous tools to L'Hôpital's rule in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether the usual limit rules for single-variable functions can be generalized to multivariable functions, specifically regarding sum, product, quotient, and composite rules.
  • Another participant clarifies that the discussion pertains to functions from Rn to R, noting that standard rules for derivatives apply to partial derivatives and the gradient vector.
  • A participant expresses concern about the application of limits at an epsilon-delta level, particularly regarding the composite rule for functions of several variables.
  • It is suggested that while derivatives maintain the epsilon-delta framework in Rm, certain theorems like the mean value theorem may not hold in the multivariable case.
  • One participant acknowledges a misunderstanding regarding the focus on derivatives, confirming that the composition law applies to limits of functions of several variables.

Areas of Agreement / Disagreement

Participants generally agree that the composition law applies to limits of multivariable functions, but there remains uncertainty about the broader applicability of limit rules and the existence of pathological cases.

Contextual Notes

Participants note the importance of epsilon-delta arguments and the potential limitations of certain theorems when extending concepts from single-variable to multivariable contexts.

Who May Find This Useful

This discussion may be of interest to students and professionals in mathematics and related fields who are exploring the properties of limits in multivariable calculus.

Diophantus
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When dealing with real valued functions (one output for now) of more than one real variable, can the usual rules from R --> R be generalised in the natural way? Specifically the sum, product, quotient and composite rules. Any pathological cases?

Also I was also wondering if there are any tools analogous to L'Hopital lying around anywhere.
 
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Looks to me like you are talking about Rn--> R: a real valued function of several values. Yes, the standard rules for derivatives apply to partial derivatives and the gradient vector.
 
Yes I know that, I was more concerned about actual limits at a pedantic epsilon-delta level. Eg. for the composite rule if f(x,y) = h(g(x,y)), and we know that

lim g(x,y) = L
(x,y)-->(a,b)

then can we always say that

lim f(x,y) = lim h(z)
(x,y)-->(a,b) z-->L
 
Derivatives *are* pedantic epsilon and delta arguments. And they all pass through without alteration in the R^m case: just replace | | with || ||, the Euclidean distance in R^m.

It is things like the mean value theorem that fail.
 
Hmm, looking back at your original post I don't know where I got the idea that you were talking about derivatives!

Yes, the "composition" law applies to limits of functions of several variables.
 
Thanks guys
 

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