Understanding the basic concept of a Limit

  • I
  • Thread starter nycmathguy
  • Start date
  • #1
nycmathguy
Summary:
Basic concept of a limit.
Hello everyone. How are you? I want to learn calculus so badly. I plan to do a self-study of calculus l, ll, and lll. Before thinking so far ahead, I need a clear, basic definition of the concept of a limit. Textbook language is never easy to grasp unless the student is gifted. I am not gifted mathematically but love the subject.

I understand the limit idea to be the following:

•The value a function is tending to without actually getting there.

•The height a function is trying to reach in terms of the y-axis.

•In terms of a point (x, y), the limit is (value the function is tending to, limit). In other words, x = the value a function is tending to and y = the limit, the height of the function.
Is my understanding of a limit clear or not?
 
  • Like
Likes Delta2

Answers and Replies

  • #2
15,136
12,863
You are sounding a bit confused. E.g. "without getting there" is wrong: ##\lim_{x \to 2} x^2=4## so the function ##f(x) =x^2## is actually getting to the limit ##4.## Things are getting a bit more complicated if infinities, or holes come into play.

##L:=\lim _{x\to a} f(x)## describes the value of ##f(x)## if ##x## approaches ##a## on some path. It does not say whether ##f(a)## is defined or not. All cases can occur: ##L=\infty \, , \,L=-\infty \, , \,L=f(a) \in \mathbb{R}## or ##f(a)## is undefined, although the limit ##L## exists (a gap in the graph of ##f## at position ##x=a##).
 
  • Like
Likes Kashmir
  • #3
Delta2
Homework Helper
Insights Author
Gold Member
4,309
1,718
I think you might confusing two different things:
  1. The value that the independent variable ##x## is tending to, which we may refer to as ##x_0##
  2. The value that the dependent variable ##y=f(x)## is tending to (this is the value the function is tending to and it is equal to the limit L)
So when we write $$\lim_{x\to 2}x^2=4$$ we mean that , AS the value of the independent variable ##x## is tending to 2 (##x_0=2## in this case), the value of the dependent variable ##y=x^2## is tending to the limit L=4.
 
Last edited:
  • Like
Likes Kashmir and nycmathguy
  • #4
nycmathguy
I think you might confusing two different things:
  1. The value that the independent variable ##x## is tending to, which we may refer to as ##x_0##
  2. The value that the dependent variable ##y=f(x)## is tending to (this is the value the function is tending to and it is equal to the limit L)
So when we write $$\lim_{x\to 2}x^2=4$$ we mean that , AS the value of the independent variable ##x## is tending to 2 (##x_0=2## in this case), the value of the dependent variable ##y=x^2## is tending to the limit L=4.

As x tends a number, y = f(x) is approaching the limit L.
 
  • #5
Delta2
Homework Helper
Insights Author
Gold Member
4,309
1,718
As x tends a number, y = f(x) is approaching the limit L.
Yes this is absolutely correct.
 
  • Like
Likes nycmathguy
  • #6
phinds
Science Advisor
Insights Author
Gold Member
17,060
8,102
In terms of a point (x, y), the limit is (value the function is tending to, limit). In other words, x = the value a function is tending to and y = the limit, the height of the function.
You want to be careful about limiting your thinking (no pun intended) to a particular axis orientation. A limit can be on either axis depending on how a function is defined and for that matter limits are not confined to 2D.
 
  • #7
nycmathguy
You are sounding a bit confused. E.g. "without getting there" is wrong: ##\lim_{x \to 2} x^2=4## so the function ##f(x) =x^2## is actually getting to the limit ##4.## Things are getting a bit more complicated if infinities, or holes come into play.

##L:=\lim _{x\to a} f(x)## describes the value of ##f(x)## if ##x## approaches ##a## on some path. It does not say whether ##f(a)## is defined or not. All cases can occur: ##L=\infty \, , \,L=-\infty \, , \,L=f(a) \in \mathbb{R}## or ##f(a)## is undefined, although the limit ##L## exists (a gap in the graph of ##f## at position ##x=a##).

This idea is totally new to me. I am going through the textbook on my own. No specific reason other than learning the material. Let's stick to the basics of calculus by avoiding math jargon and complicated symbols that mean nothing to a novice calculus self-study individual.

Here is a better definition:

As x tends a number, y = f(x) is approaching the limit L.

You say?
 
  • #8
nycmathguy
You want to be careful about limiting your thinking (no pun intended) to a particular axis orientation. A limit can be on either axis depending on how a function is defined and for that matter limits are not confined to 2D.

This idea is totally new to me. I am going through the textbook on my own. No specific reason other than learning the material. Let's stick to the basics of calculus by avoiding math jargon and complicated symbols that mean nothing to a novice calculus self-study individual.

Here is a better definition:

As x tends a number, y = f(x) is approaching the limit L.

You say?
 
  • #9
Delta2
Homework Helper
Insights Author
Gold Member
4,309
1,718
You want to be careful about limiting your thinking (no pun intended) to a particular axis orientation. A limit can be on either axis depending on how a function is defined and for that matter limits are not confined to 2D.
He might me limiting his thinking, but you expanding it too much, by start talking about multivariable calculus when the OP said he just starting learning calculus I.
 
  • #11
Delta2
Homework Helper
Insights Author
Gold Member
4,309
1,718
I say that doesn't even make sense
this is some sort of overstatement by you. I read the first link and I find that what the OP says at #8 is very similar to what the link says. Why do you say it doesn't even make sense?
 
  • #12
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
17,568
9,339
this is some sort of overstatement by you. I read the first link and I find that what the OP says at #8 is very similar to what the link says. Why do you say it doesn't even make sense?
Because it should be:

If as ##x## tends to ##a##, ##f(x)## tends to ##L##, then we say that ##L## is the limit of the function ##f(x)## as ##x## tends to ##a##; and, we write: $$\lim_{x \rightarrow a}f(x) = L$$ Trying to improve on this is a mathematical dead-end, IMO.
 
  • Like
Likes MidgetDwarf
  • #13
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
17,568
9,339
I understand the limit idea to be the following:

•The value a function is tending to without actually getting there.
There's nothing in the definition of a limit that says that it doesn't "actually get there".

First, if we have a constant function ##f(x) = c##, then we have (for any ##a##) $$\lim_{x \rightarrow a} f(x) = c$$ and we see that the limit is attained.

Also, we have: $$\lim_{x \rightarrow 0} x \sin \frac 1 x = 0$$ and the function ##f(x) = x \sin \frac 1 x## attains the limit value ##0## an infinite numbers of times for ##x \ne 0##.
 
  • #14
Delta2
Homework Helper
Insights Author
Gold Member
4,309
1,718
Because it should be:

If as ##x## tends to ##a##, ##f(x)## tends to ##L##, then we say that ##L## is the limit of the function ##f(x)## as ##x## tends to ##a##; and, we write: $$\lim_{x \rightarrow a}f(x) = L$$ Trying to improve on this is a mathematical dead-end, IMO.
I must be brain dead cause I also find what you saying here very similar to what the OP says at #8, you just expand it a bit further.
 
  • #15
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
17,568
9,339
I must be brain dead cause I also find what you saying here very similar to what the OP says at #8, you just expand it a bit further.
Have a cup of coffee and read them both again!
 
  • Haha
Likes Delta2
  • #16
nycmathguy
There's nothing in the definition of a limit that says that it doesn't "actually get there".

First, if we have a constant function ##f(x) = c##, then we have (for any ##a##) $$\lim_{x \rightarrow a} f(x) = c$$ and we see that the limit is attained.

Also, we have: $$\lim_{x \rightarrow 0} x \sin \frac 1 x = 0$$ and the function ##f(x) = x \sin \frac 1 x## attains the limit value ##0## an infinite numbers of times for ##x \ne 0##.
I thank you for your reply. However, I am looking at calculus for the first time outside of a classroom setting. I am slowly going through the textbook one chapter, one section at a time. The limit of trig functions is still a few chapters and sections ahead.
 
  • Like
Likes Delta2
  • #17
15,136
12,863
I thank you for your reply. However, I am looking at calculus for the first time outside of a classroom setting. I am slowly going through the textbook one chapter, one section at a time. The limit of trig functions is still a few chapters and sections ahead.
Draw some graphs of possible functions and look at what happens to the graph if ##x## approaches certain points. E.g.

1623500170881.png
 
  • Like
Likes nycmathguy
  • #18
12,951
6,798
For more details on limits checkout the 3blue1brown youtbe sequence called the Essence of Calculus. Its ten short 10min videos covering this and other key concepts.
 
  • Like
Likes The Fez and fresh_42
  • #19
nycmathguy
Draw some graphs of possible functions and look at what happens to the graph if ##x## approaches certain points. E.g.

View attachment 284372
We will get to graphs as I journey through the textbook. I'm excited about calculus events my PSA level is 4.9. Pray for me.
 
  • Like
Likes fresh_42 and Delta2
  • #20
berkeman
Mentor
60,016
10,224
my PSA level is 4.9
That's a bit high for your age. Are you under a doctor's care for it?
 
  • #21
nycmathguy
That's a bit high for your age. Are you under a doctor's care for it?

Yes, my primary care doctor at the VA Hospital wants me back in 6 weeks for another PSA. There are men with higher PSA levels that do not have cancer. I am not going to quickly run into a prostate biopsy situation. Let's see what happens in 6 weeks. I'll keep you posted. Thank you so much for asking.
 
  • Like
Likes Delta2, berkeman and fresh_42
  • #22
508
181
This video, and channel overall, should help you on your journey (patrickJMT has some of the BEST videos on computational calculus IMO):
 
  • Like
Likes nycmathguy and Delta2
  • #23
nycmathguy
This video, and channel overall, should help you on your journey (patrickJMT has some of the BEST videos on computational calculus IMO):

Thanks for the video clip. I will check it out.
 
  • #24
mathwonk
Science Advisor
Homework Helper
2020 Award
11,184
1,382
I think I understand where you are coming from with the "without getting there" language because I used to actualky use that in teaching limits, although I sympathize with opponents of it.

Take an example of a limit if a sequence, (which is a function defined on the positive integers), like f(n) = 1/n. We ask for "the limit of f(n) as n goes to infinity". To me, the entries in the sequence are approximating values, and the limit is the number they are trying to approximate. So ask yourself what number do the numbers 1/n get closer to as n gets larger and larger. The answer is zero. That is, if you go out far enough in the sequence, the elements get, and remain, as close to zero as you might wish. I.e. if you want them to be closer than .0001, then after n gets larger than 10,000, every later sequence entry 1/n will be closer to zero than .0001. So zero is the limit of the sequence. In this case no element of the sequence ever equals zero, so although no entry in the sequence actually gets to zero, every element, far enough out, gets as close as desired.

Now here is another sequence: f(1) =1, f(2) = 2, f(3) = 3, f(4) = 4, f(5)= 1/5, f(6) = 1/6, f(7) =0, f(8) = 0, f(9) = 0, ....f(n) = 0 for every n ≥ 9. This sequence fools around for a while getting larger, but soon turns around and starts getting smaller, and all of a sudden jumps to zero, where it remains forever after. Since the only thing that matters in a sequence is where it eventually goes, this sequence is also (eventually) approximating to, i.e. heding ultimtely toward, zero. Indeed if you wnt to get an approximation to within .000001, of zero, you can take any sequence entry further out than f(7). So here again the limit is zero. But in this case, most elements of the sequence do actually get to zero, the limit.

So sometimes the sequence actualy gets to the limit "early", and sometimes it does not. This doies not matter at all for the notion of a limit. The limit is the number that the sequence ultimately zeroes in on, whether it actually gets there earlier or not does not matter.

I have to go now, but the case of functions is similar. I.e. when looking at the limit of a function like f(x) = (x^2-4)/(x-2), as x goes to 2, we are interested in what number is being approximated by these values f(x) when x is very near 2, but not equal to 2. That is because the function is not defined at 2. This is analogous to the fact the sequence function above was not defined at n = infinity.

It is more confusing to consider functions like f(x) = x^2, and ask for the ,imit as goes to 2, because this function is defined at 2, i.e. f(2) = 4. Since the limit of this function as x goes to 2, is also 4, you might say that the function does get to its limit, namely the limit as x goes to 2, is whatever number x^2 is fgettoing closer to as x gets closer to 2, and this is 4. But you are supposed to compute the limit only by looking at numbers x with x ≠ 2, so really, in my opinion this function also does not "get to 4", at least not as long as x has not got to 2.

The link is that some functions are defined at the point that x is going to, i.e. this f(x) = x^2 is defined at x=2, and one can ask two separate questions: 1) is there a limit (and what is it), of x^2, as x goes to 2 without reaching it? 2) does that limit, equal the value of f(2)? If so, the function is a good function, called continuous.

got to go.
 
  • Like
Likes nycmathguy
  • #25
nycmathguy
I think I understand where you are coming from with the "without getting there" language because I used to actualky use that in teaching limits, although I sympathize with opponents of it.

Take an example of a limit if a sequence, (which is a function defined on the positive integers), like f(n) = 1/n. We ask for "the limit of f(n) as n goes to infinity". To me, the entries in the sequence are approximating values, and the limit is the number they are trying to approximate. So ask yourself what number do the numbers 1/n get closer to as n gets larger and larger. The answer is zero. That is, if you go out far enough in the sequence, the elements get, and remain, as close to zero as you might wish. I.e. if you want them to be closer than .0001, then after n gets larger than 10,000, every later sequence entry 1/n will be closer to zero than .0001. So zero is the limit of the sequence. In this case no element of the sequence ever equals zero, so although no entry in the sequence actually gets to zero, every element, far enough out, gets as close as desired.

Now here is another sequence: f(1) =1, f(2) = 2, f(3) = 3, f(4) = 4, f(5)= 1/5, f(6) = 1/6, f(7) =0, f(8) = 0, f(9) = 0, ....f(n) = 0 for every n ≥ 9. This sequence fools around for a while getting larger, but soon turns around and starts getting smaller, and all of a sudden jumps to zero, where it remains forever after. Since the only thing that matters in a sequence is where it eventually goes, this sequence is also (eventually) approximating to, i.e. heding ultimtely toward, zero. Indeed if you wnt to get an approximation to within .000001, of zero, you can take any sequence entry further out than f(7). So here again the limit is zero. But in this case, most elements of the sequence do actually get to zero, the limit.

So sometimes the sequence actualy gets to the limit "early", and sometimes it does not. This doies not matter at all for the notion of a limit. The limit is the number that the sequence ultimately zeroes in on, whether it actually gets there earlier or not does not matter.

I have to go now, but the case of functions is similar. I.e. when looking at the limit of a function like f(x) = (x^2-4)/(x-2), as x goes to 2, we are interested in what number is being approximated by these values f(x) when x is very near 2, but not equal to 2. That is because the function is not defined at 2. This is analogous to the fact the sequence function above was not defined at n = infinity.

It is more confusing to consider functions like f(x) = x^2, and ask for the ,imit as goes to 2, because this function is defined at 2, i.e. f(2) = 4. Since the limit of this function as x goes to 2, is also 4, you might say that the function does get to its limit, namely the limit as x goes to 2, is whatever number x^2 is fgettoing closer to as x gets closer to 2, and this is 4. But you are supposed to compute the limit only by looking at numbers x with x ≠ 2, so really, in my opinion this function also does not "get to 4", at least not as long as x has not got to 2.

The link is that some functions are defined at the point that x is going to, i.e. this f(x) = x^2 is defined at x=2, and one can ask two separate questions: 1) is there a limit (and what is it), of x^2, as x goes to 2 without reaching it? 2) does that limit, equal the value of f(2)? If so, the function is a good function, called continuous.

got to go.

Interesting. Thank you.
 

Related Threads on Understanding the basic concept of a Limit

  • Last Post
Replies
9
Views
969
Replies
7
Views
3K
  • Last Post
Replies
7
Views
2K
  • Last Post
Replies
2
Views
842
Replies
12
Views
3K
Replies
6
Views
1K
  • Last Post
5
Replies
100
Views
4K
Replies
17
Views
1K
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
1
Views
2K
Top