Can a Function Have a Limit of 0 and Derivative of 1 at Infinity?

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Discussion Overview

The discussion centers on the possibility of a smooth function having a limit of 0 and a derivative of 1 as x approaches infinity. Participants explore the implications of these conditions on the behavior of the function and its derivative, engaging in theoretical reasoning and mathematical exploration.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant questions whether a smooth function can exist such that \(\lim_{x \to \infty} y(x) = 0\) and \(\lim_{x \to \infty} y'(x) = 1\).
  • Another participant suggests that for a smooth function to have a limit at infinity, the derivative should approach 0, positing that the function must become progressively flatter as it approaches its limit.
  • A third participant mentions the Mean Value Theorem (MVT) as a basis for their reasoning regarding the behavior of derivatives in relation to limits.
  • One participant reflects on their struggle with the formal definitions involving epsilons and deltas while attempting to understand the problem more rigorously.
  • Another participant provides a counterexample with the function \(\frac{\sin(x^2)}{x}\), which approaches 0 while its derivative oscillates, suggesting that the derivative does not necessarily approach 0 and cannot tend to a nonzero number, referencing MVT.
  • The same participant corrects their earlier statement about a different function, \(\frac{\sin(x^2)}{\sqrt{x}}\), noting that its derivative oscillates unboundedly while the function itself approaches 0.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between the limits of a function and its derivative at infinity. There is no consensus on whether such a function can exist, and the discussion remains unresolved.

Contextual Notes

Participants acknowledge the complexity of the problem, with references to the Mean Value Theorem and the challenges of applying formal definitions. There are indications of missing assumptions and the need for rigorous proofs.

zetafunction
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can exist an smooth function with the property

[tex]y(\infty) =0[/tex] and [tex]y'(\infty) =1[/tex] ?

the inverse case, a function that tends to 1 for big x and whose derivative tends to 0 is quite obvious but this case i am not sure if there will exist
 
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I suppose that you mean
[tex]\lim_{x \to \infty} y(x) = 0[/tex] and [tex]\lim_{x \to \infty} y'(x) = 1[/tex] ?

Actually, I think that for a smooth function to have a limit at infinity, the derivative should have limit 0 (at least that's what my intuition tells me: for the function to have a limit at infinity, it should become progressively more flat, so it doesn't run away from its limit value).

I have some other work now, but I will try to prove that rigorously later (if you want, give it a try yourself).
 
It follows easily from MVT.
 
That's what I figured, but I got caught up in epsilons and deltas on the back of my scrap piece of paper.
After finishing my Saturday's to-do list I will take a completely blank paper of normal size and try again :)
 
On the second thought, derivative doesn't have to go to zero, unfortunetely. Consider [tex]\frac{\sin (x^2)}{x}[/tex]. It clearly tends to zero, yet the derivative oscilates. Still, the derivative cannot tend to a nonzero number, and this follows from MVT for sure :wink:
Sorry for the mistake.

Edit: derivative of [tex]\frac{\sin (x^2)}{\sqrt{x}}[/tex] oscilates unboundedly, while the function goes to zero.
 
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