What is the function that describes this Asymptotic behaviour?

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Discussion Overview

The discussion revolves around finding a function \( a(x) \) that exhibits specific asymptotic behavior, namely \( a(x) \rightarrow 1 \) as \( x \gg x_c \) and \( a(x) \rightarrow f(x) \) as \( x \ll x_c \). Participants explore various forms of \( a(x) \) and the implications of different choices for \( f(x) \). The scope includes mathematical reasoning and exploratory function formulation.

Discussion Character

  • Exploratory, Mathematical reasoning, Debate/contested

Main Points Raised

  • Some participants propose defining \( a(x) \) piecewise, with \( a(x) = f(x) \) for \( x \leq x_c \) and \( a(x) = 1 \) for \( x > x_c \).
  • Others emphasize the desire for the simplest possible function \( a(x) \), noting that there could be infinitely many functions that satisfy the asymptotic conditions.
  • A participant suggests a specific form \( a(x) = \frac{f(x)x_c - x}{x_c - x} \) and notes it meets the asymptotic requirements.
  • Another participant introduces the function \( a(x) = \frac{2 \arctan(x)}{\pi} \) as a potential candidate.
  • Concerns are raised about the dependence of \( a(x) \) on the unspecified function \( f(x) \) and the implications of \( f(x) \) being bounded or continuous.
  • There is discussion about the behavior of \( f(x) \) and its potential forms, including constants or linear dependencies on \( x \).
  • Participants express uncertainty about how variations in \( f(x) \) could affect the overall behavior of \( a(x) \) and the asymptotic relationships.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a single function \( a(x) \) that satisfies the conditions, and multiple competing views and proposed functions remain in the discussion.

Contextual Notes

Participants acknowledge that the behavior of \( a(x) \) is heavily influenced by the unspecified function \( f(x) \), and there are assumptions about the continuity and boundedness of \( f(x) \) that have not been fully explored.

Arman777
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I would like to find a function such that for

$$a(x) \rightarrow 1~\text{for}~(x \gg x_c)$$

$$a(x) \rightarrow f(x)~\text{for}~(x \ll x_c)$$

What could be the ##a(x)## ? I have tried some simple functions but could not figure it out. Maybe I am just blind to see the correct result.
 
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Arman777 said:
I would like to find a function such that for

$$a(x) \rightarrow 1~\text{for}~(x \gg x_c)$$

$$a(x) \rightarrow f(x)~\text{for}~(x \ll x_c)$$

What could be the ##a(x)## ? I have tried some simple functions but could not figure it out. Maybe I am just blind to see the correct result.
This is far too vague to give a reasonable answer. How about
$$
a(x):=\begin{cases} f(x)&\text{ if }x\leq x_c\\1&\text{ if }x> x_c \end{cases}
$$
 
fresh_42 said:
This is far too vague to give a reasonable answer.
How so ?
fresh_42 said:
How about
Hmm..that also works I guess
 
What if I say, finding simplest possible $a(x)$. Is that makes sense ? Of course there could be infinetly many functions, but I am looking for least complicated/simplest one
 
Arman777 said:
What if I say, finding simplest possible $a(x)$. Is that makes sense ? Of course there could be infinetly many functions, but I am looking for least complicated/simplest one
The one that @fresh_42 suggested is pretty simple. Without knowing what f(x) is, you probably won't find anything simpler than that.
 
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No, I am looking for one single function, I mean. When I take the asymptotic behavior, it produces the results I have mentioned.

I am not looking for the same answer in a different form.
 
Do you want it to be a continuous function of x?
Consider this:
calculate ##r = (\arctan(x)/\pi + 0.5)##
calculate ##h(x) = f(x)(1-r) + r##
 
Last edited:
Okay, I have found. It seems the answer is

For $$a(x) = \frac{f(x)x_c - x}{x_c - x}$$
$$a(x) = 1~\text{for}~x \gg x_c$$
$$a(x) = f(x)~\text{for}~x \ll x_c$$
 
Try ##a(x)=\frac{2.arctan(x)}{\pi}##
 
  • #10
Arman777 said:
Okay, I have found. It seems the answer is

For $$a(x) = \frac{f(x)x_c - x}{x_c - x}$$
$$a(x) = 1~\text{for}~x \gg x_c$$
$$a(x) = f(x)~\text{for}~x \ll x_c$$
Still heavily depends on ##f(x)##. And what do you mean by ##x\ll x_c##? What if ##x\to -\infty ##?

This doesn't solve your problem unless you make several assumptions on ##f(x)## which you didn't tell us about!
 
  • #11
Well, currently we don't know the ##f(x)## but we know that it does not contain ##x_c##. BTW ##x_c## is just a constant, and ##x## ranges from ##[0, \infty)##. I am not interested the behaviour of the ##f(x)## as ##x \ll x_c## I am just interested in the behaviour of the ##a(x)## as a whole. ##f(x)## could also be a some constant. Or linearly dependent on ##x## but its not much complicated...

I guess you mean that if we open up the ##f(x)##, the behavior of the asymptotes will change?
 
  • #12
Arman777 said:
I guess you mean that if we open up the ##f(x)##, the behavior of the asymptotes will change?

Yes. I haven't checked in detail all possibilities, but it seemed you assumed that ##f(x)## is bounded in some sense and continuous. Otherwise, it could "overwrite" the rest of the formula.
 
  • #13
fresh_42 said:
Yes. I haven't checked in detail all possibilities, but it seemed you assumed that ##f(x)## is bounded in some sense and continuous. Otherwise, it could "overwrite" the rest of the formula.
Well yes its. Also it seems that the f(x) must be in the 0th order or 1st order. Otherwise the asymptotic relation fails. But of course this is true for this function. I am not sure we can find a more general one
 
  • #14
Arman777 said:
Okay, I have found. It seems the answer is

For $$a(x) = \frac{f(x)x_c - x}{x_c - x}$$
$$a(x) = 1~\text{for}~x \gg x_c$$
$$a(x) = f(x)~\text{for}~x \ll x_c$$
You might like to play around with
https://www.desmos.com/calculator/da9wzphuxc

[You can rescale the axes by shift-drag near each axis.]

1632813294811.png
 

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