Need for formal definition of limits

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Discussion Overview

The discussion revolves around the necessity and understanding of the formal definition of limits in mathematics, particularly the epsilon-delta definition. Participants explore the implications of this definition for both single-variable and multi-variable functions, as well as its role in teaching continuity and limits.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the necessity of the epsilon-delta definition of limits, suggesting that the intuitive understanding of right and left-hand limits should suffice.
  • Another participant challenges this view by asking how one would define right-handed and left-handed limits without using epsilon and delta.
  • A participant introduces a specific function to illustrate the complexity of limits, especially in cases involving irrational numbers and multi-variable functions.
  • Concerns are raised about the meaning and purpose of the epsilon-delta correspondence, specifically regarding the constraints it places on the differences between function values and limits.
  • One participant emphasizes the importance of the formal definition for teaching continuity and ensuring that functions behave as expected, arguing that it is a well-established conclusion rather than an arbitrary choice.
  • A suggestion is made that attempting to define or prove concepts in one's own way can enhance understanding of the formal definitions and proofs.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and clarity of the epsilon-delta definition of limits. Some argue for its importance in formal mathematics, while others question its practicality and relevance to intuitive understanding.

Contextual Notes

The discussion highlights the complexity of limits in various contexts, including single-variable and multi-variable functions, and the potential challenges in teaching these concepts without a formal framework.

Muhammad Ali
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I could not understand the need for the formal definition of limits. When we know that the limit of a function 'f' exists at a point say 'a' if the the function 'f' has both right and left hand limits exist at that point 'a' and equal. Then what is the need for the formal definition, namely delta-epsilon definition? Why is the life for the students made difficult? I hope for a comprehensive answer.
 
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Well, how would you define "right-handed" and "left-handed" limits WITHOUT the epsilon and deltas? :smile:
 
I don't know.
 
How would you apply "left" and "right" limits to this function:

f(x)= 0 if x is irrational, 1/n if x= m/n reduced to lowest terms

What is its limit as x approaches some number a?

In addition, the situation gets a lot more complicated with functions of more than one real variable, not to mention functions of complex variables. "Left" and "right" no longer apply.
 
Ok, but I don't understand what we are getting by establishing a correspondence between epsilon and delta.
or
What is the purpose of constraining the difference of the function 'f(x)' and its limit say 'L' less than epsilon or similarly the difference of any x and 'a' less than delta? What is the meaning of the last sentence?
 
Has no one drawn you a picture? Continuous, naively, means that it doesn't tear things apart. The formal interpretation of this statement is that the epsilon-delta stuff - if things start sufficiently close together (delta) then they cannot be pulled vary far apart (epsilon).

If we don't have the formal definition then how can we ever check that something is continuous? How can we teach it? It is an exact statement. It took years for people to reach the conclusion that this was the definition we wanted - it is neither God given, nor plucked from imagination.
 
A good method for understanding definitions or proofs is to try to define or proove something in your own way, which doesn't work, so it can lead to a better understanding of the given definitions or proofs.
 

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