A definitions for the terms the limit does not exists

In summary, the limit does not exist if either the limit goes to positive or negative infinity, or if the limit is not unique. Another definition for the limit going to infinity is if for every N, there is an M such that for all x greater than M, f(x) is greater than N. Similarly, the limit does not exist if for all t, there is some e>0 such that for all d>0, there exists an x closer to x0 than d, but with f(x) further from t than e.
  • #1
quasar987
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a definitions for the terms "the limit does not exists"

Since my textbook doesn't have a definitions for the terms "the limit does not exists" and "the limit goes to infinity", I tried to make them up. I'd like to know if they're correct.

1) Consider [itex] f: \mathcal{D}\longrightarrow \mathbb{R}[/itex] a function and [itex]x_0[/itex] an accumulation point of [itex]\mathcal{D}[/itex]. We say that the limit as x approaches [itex]x_0[/itex] goes to positive infinity (resp. negative infinity) if [itex]\forall M \in \mathbb{R}, \ \exists \delta>0[/itex] such that [itex]x \in \mathcal{D} \cap V'(x_0,\delta) \Rightarrow f(x)>M[/itex] (resp.[itex]f(x)<M[/itex]), and we write

[tex]\lim_{x \rightarrow x_0} f(x) = +\infty \ (\mbox{resp. -\infty})[/tex]

2) Consider [itex] f: \mathcal{D}\longrightarrow \mathbb{R}[/itex] a function. If [itex]\mathcal{D}[/itex] is unbounded superiorly (?) (i.e. has no upper bound), we say that the limit as x approaches positive infinity goes to positive infinity (resp. negative infinity) if [itex]\exists N \in \mathbb{R}[/itex] such that [itex]\forall x \in \mathcal{D}, \ x>N \Rightarrow f(x)[/itex] is strictly increasing (resp. strictly decreasing). In other words, we say that the limit as x approaches positive infinity goes to positive infinity (resp. negative infinity) if [itex]\exists N \in \mathbb{R}[/itex] such that [itex]\forall y, z \in \mathcal{D}[/itex] and [itex] y, z>N, \ z>y \Rightarrow f(z)>f(y)[/itex] (resp.[itex]f(z)<f(y)[/itex]), and we write

[tex]\lim_{x \rightarrow \+\infty} f(x) = +\infty \ (\mbox{resp. -\infty})[/tex]


3) We have an analogous definition for the limit as x goes to negative infinity if the domain has no lower bound. And finally,...

4) Consider [itex] f: \mathcal{D}\longrightarrow \mathbb{R}[/itex] a function and [itex]x_0[/itex] an accumulation point of [itex]\mathcal{D}[/itex]. We say that the limit as x approaches [itex]x_0[/itex], [itex]+\infty[/itex] or [itex]-\infty[/itex] (whichever applies) does not exists if either

i) the limit goes to [itex]+\infty[/itex].

ii) the limit goes to [itex]-\infty[/itex].

iii) the limit is not unique.


Also, if you can think of another definition, or a caracterisation that would make the proofs easier, I'd be very interested to hear it.

Mmh, I can think of one for definition 2 and 3: For 2) "blah, blah" iif for all sequences [itex]\{x_n\}[/itex] such that [itex]x_n \in \mathcal{D}[/itex] and [itex]\{x_n\}[/itex] is strictly increasing for at least all n greater than a certain [itex]N \in \mathbb{R}[/itex] that has [itex]+\infty[/itex] for a limit, the corresponding sequence [itex]\{f(x_n)\}[/itex] has [itex]+\infty[/itex] (resp. [itex]-\infty[/itex]) for a limit. The caracterisation for 3 is analogous.

Phew, this took 45 minutes to write! :cry:

Thanks for your replies.
 
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  • #2
1) is correct except that you omitted to say the "limit of what?"

2) looks wrong. i.e it differs greatly from the case in 1) which ti should resemble closely. i.e. the limit of f(x) as x goes to plus infinity, equals plus infinity iff, for every N, there is an M, such that for all x larger than M, f(x) is larger than N.

3) The definition of does nor exist also oooks highly suspicious. In general just negate the rpevious statements.

e.g. the limit of f(x) as x goes to x0 does not exist iff, for all t, there is some e>0, such that for all d > 0, there exists and x closer to x0 than d, and yet with f(x) further from t than e.
 
  • #3
IOW, start with the epsilon-delta way to say "the limit exists" and negate it.
 

1. What does "the limit does not exist" mean?

The limit does not exist refers to a mathematical concept where the value of a function or sequence cannot be determined at a certain point, usually due to a discontinuity or an infinite oscillation.

2. How do you determine if the limit does not exist?

The limit does not exist if the left-hand limit (approaching from the left) and the right-hand limit (approaching from the right) of a function or sequence are not equal at a certain point, or if either one does not exist.

3. Can a limit not exist at a single point?

Yes, a limit can fail to exist at a single point. This can happen if the function or sequence has a jump discontinuity, an infinite discontinuity, or an oscillating behavior at that point.

4. What does it mean when a limit is undefined?

When a limit is undefined, it means that the value of the function or sequence at that point cannot be determined. This can happen when the function or sequence is not defined at that point, or when there is a hole in the graph at that point.

5. What is the difference between a limit not existing and being undefined?

The main difference is that a limit not existing means that the left-hand and right-hand limits do not agree, while being undefined means that the value of the function or sequence cannot be determined at that point. In other words, a limit not existing indicates a discontinuity, while being undefined indicates a gap in the graph.

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