Can these limits be proven to be equal?

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Discussion Overview

The discussion centers around the limits of the function \( \frac{1}{x} \) as \( x \) approaches 0 from the left and right. Participants explore whether the limits \( \lim_{a \to 0^-} \frac{1}{a} \) and \( \lim_{b \to 0^+} \frac{1}{b} \) are equal, examining both intuitive and rigorous approaches to proving or disproving this statement.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that the limits are not equal, citing the behavior of the function \( \frac{1}{x} \) approaching positive infinity from the right and negative infinity from the left.
  • Others propose that a rigorous proof could be constructed using the Cauchy definition of limits, suggesting a method to demonstrate the divergence of the limits without relying on graphical representations.
  • A participant outlines a general approach for proving limits that approach infinity, indicating the need for adjustments when dealing with one-sided limits.

Areas of Agreement / Disagreement

Participants generally disagree on the equality of the limits, with some asserting they are not equal and others exploring the possibility of a rigorous proof for this assertion.

Contextual Notes

Participants mention the need for careful handling of left- and right-side limits and the implications of limits approaching positive and negative infinity, indicating that the discussion may depend on specific definitions and assumptions about limits.

Bipolarity
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Prove that
[tex]\lim_{a→0-}\frac{1}{a} = \lim_{b→0+}\frac{1}{b}[/tex]

Is this statement true? How can one prove its truth/falsity? Would we need to use the precise Cauchy definition of the limit to do this?

BiP
 
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Bipolarity said:
Prove that
[tex]\lim_{a→0-}\frac{1}{a} = \lim_{b→0+}\frac{1}{b}[/tex]

Is this statement true?
No.
Bipolarity said:
How can one prove its truth/falsity?
A quick sketch of the graph of y = 1/x should convince you that this statement is not true.
Bipolarity said:
Would we need to use the precise Cauchy definition of the limit to do this?

BiP
 
Mark44 said:
No.A quick sketch of the graph of y = 1/x should convince you that this statement is not true.

So I see that from one end it approaches positive infinity, and from the other end it approaches negative infinity.

But is there a way to prove this rigorously without having to refer to a visual aid such as a graph?

BiP
 
Yes, in the same way that you prove that the limit as x → a of a function is infinity; for each M > 0, there is a ## \delta > 0## such that if |x - a| < ## \delta##, then f(x) > M.

You have to adjust things slightly to deal with left- and right-side limits, and dealing with a limit of negative infinity, but this is the general idea.
 

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