Limits of Infinity: Does f(x) Exist?

In summary, the concept of limit as x approaches infinity does not necessarily imply that the limits at positive and negative infinity must exist and be equal. This can be seen with the function f(x) = Arctan(x). Additionally, the limit of (2x^2 + 3)^1/2 / (4x + 2) as x approaches positive infinity is equal to 1/4, which contradicts the statement made in I.A. Maron's calculus text. It is important to be aware of conventions and definitions when working with limits at infinity.
  • #1
phymatter
131
0
Is it so that for limx->infinity f(x) to exist ,
limx->+infinity f(x) and limx->-infinity should exist and be equal ? if so then why ?
 
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  • #2
phymatter said:
Is it so that for limx->infinity f(x) to exist ,
limx->+infinity f(x) and limx->-infinity should exist and be equal ? if so then why ?

No, it isn't so. Look at f(x) = Arctan(x).
 
  • #3
LCKurtz said:
No, it isn't so. Look at f(x) = Arctan(x).

Are you sure , because i.a. maron single variable calculus says that limx->infinity (2x2+3)1/2/(4x+2) does not exist for the same reason .
 
  • #4
[tex] \lim_{x\to \infty}[/tex] IS the limit as x goes to positive infinity.
 
  • #5
phymatter said:
Are you sure , because i.a. maron single variable calculus says that limx->infinity (2x2+3)1/2/(4x+2) does not exist for the same reason .

[tex]\lim_{x \to \infty}\frac{\sqrt{2x^2 + 3}}{4x + 2} = \frac{1}{4}[/tex]

Is I.A. Maron the author of a calculus text? I never heard of it.
 
  • #6
You have to be careful with your conventions. Normally, with the real numbers, "[itex]\infty[/itex]" means "[itex]+\infty[/itex]. However, with the "one-point compactification" of the real numbers (not commonly used for Calculus problems!) we have just the one "point at infinity".
 

FAQ: Limits of Infinity: Does f(x) Exist?

What is the definition of the limit of infinity?

The limit of infinity is a concept in calculus that refers to the behavior of a function as the input approaches positive or negative infinity. It represents the value that a function approaches, but may never reach, as the input increases or decreases without bound.

How do you determine if a limit of infinity exists?

To determine if a limit of infinity exists, you must evaluate the function at values that approach infinity from both positive and negative directions. If both of these values approach the same value, then the limit of infinity exists. If the two values approach different values or do not approach a value at all, then the limit of infinity does not exist.

Can the limit of infinity be a real number?

No, the limit of infinity is not a real number. It represents the behavior of a function as the input approaches infinity, but it is not an actual value that the function reaches.

What is the difference between a limit of infinity and a limit at a finite number?

The limit of infinity represents the behavior of a function as the input approaches infinity, while a limit at a finite number represents the behavior of a function at a specific input value. In other words, the limit of infinity is concerned with the overall trend of the function as the input increases or decreases without bound, while a limit at a finite number is concerned with the behavior of the function at a particular input value.

Can a function have a limit of infinity at multiple points?

Yes, a function can have a limit of infinity at multiple points. This means that as the input approaches positive or negative infinity, the function approaches a certain value at multiple points. However, it is also possible for a function to have a limit of infinity at one point but not at another point.

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