Difference between revisions of "The Limit of a Function"

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[[File:Epsilon-delta.png|thumb|500px|right|Whenever a point <math>x</math> is within <math>\delta</math> units of <math>c</math> , <math>f(x)</math> is within <math>\varepsilon</math> units of <math>L</math>]]
  
 +
In preliminary calculus, the concept of a limit is probably the most difficult one to grasp (after all, it took mathematicians 150 years to arrive at it); it is also the most important and most useful one.
  
* [https://mathresearch.utsa.edu/wikiFiles/MAT1214/Limits%20of%20Functions/MAT1214_2.2TheLimitOfAFunctionPwPt.pptx The Limit of a Function] PowerPoint file created by Dr. Sara Shirinkam, UTSA.
+
The intuitive definition of a limit is inadequate to prove anything rigorously about it. The problem lies in the vague term "arbitrarily close". We discussed earlier that the meaning of this term is that the closer <math>x</math> gets to the specified value, the closer the function must get to the limit, so that however close we want the function to the limit, we can accomplish this by making <math>x</math> sufficiently close to our value. We can express this requirement technically as follows:
 +
 
 +
==Formal definition of a limit==
 +
Let <math>f(x)</math> be a function defined on an open interval <math>D</math> that contains <math>c</math> , except possibly at <math>x=c</math> . Let <math>L</math> be a number. Then we say that
 +
:<math>\lim_{x\to c}f(x)=L</math>
 +
if, for every <math>\varepsilon>0</math> , there exists a <math>\delta>0</math> such that for all <math>x\in D</math> with
 +
:<math>0<|x-c|<\delta</math>
 +
we have
 +
:<math>\Big|f(x)-L\Big|<\varepsilon</math>
 +
 
 +
To further explain, earlier we said that "however close we want the function to the limit, we can find a corresponding <math>x</math> close to our value." Using our new notation of epsilon (<math>\varepsilon</math>) and delta (<math>\delta</math>), we mean that if we want to make <math>f(x)</math> within <math>\varepsilon</math> of <math>L</math> , the limit, then we know that making <math>x</math> within <math>\delta</math> of <math>c</math> puts it there.
 +
 
 +
Again, since this is tricky, let's resume our example from before: <math>f(x)=x^2</math> , at <math>x=2</math>. To start, let's say we want <math>f(x)</math> to be within .01 of the limit. We know by now that the limit should be 4, so we say: for <math>\varepsilon=0.01</math> , there is some <math>\delta</math> so that as long as <math>0<|x-c|<\delta</math> , then <math>\Big|f(x)-L\Big|<\varepsilon</math> .
 +
 
 +
To show this, we can pick ''any'' <math>\delta</math> that is bigger than 0, so long as it works. For example, you might pick <math>10^{-14}</math> , because you are absolutely sure that if <math>x</math> is within <math>10^{-14}</math> of 2, then <math>f(x)</math> will be within <math>0.01</math> of 4. This <math>\delta</math> works for <math>\varepsilon=0.01</math> . But we can't just pick a specific value for <math>\varepsilon</math> , like 0.01, because we said in our definition "for '''every''' <math>\varepsilon>0</math> ." This means that we need to be able to show an infinite number of <math>\delta</math>s, one for each <math>\varepsilon</math> . We can't list an infinite number of <math>\delta</math>s!
 +
 
 +
Of course, we know of a very good way to do this; we simply create a function, so that for every <math>\varepsilon</math> , it can give us a <math>\delta</math> . In this case, one definition of <math>\delta</math> that works is
 +
<math>\delta(\varepsilon)=\left\{\begin{matrix}2\sqrt2-2&\mbox{if }\epsilon\ge4\\\sqrt{\epsilon+4}-2&\mbox{if }\epsilon<4\end{matrix}\right.</math>
 +
 
 +
So, in general, how do you show that <math>f(x)</math> tends to <math>L</math> as <math>x</math> tends to <math>c</math>? Well imagine somebody gave you a small number <math>\varepsilon</math> (e.g., say <math>\varepsilon=0.03</math>). Then you have to find a <math>\delta>0</math> and show that whenever <math>0<|x-c|<\delta</math> we have <math>\Big|f(x)-L\Big|<0.03</math> . Now if that person gave you a smaller <math>\varepsilon</math> (say <math>\varepsilon=0.002</math>) then you would have to find another <math>\delta</math>, but this time with 0.03 replaced by 0.002. If you can do this for ''any'' choice of <math>\varepsilon</math> then you have shown that <math>f(x)</math> tends to <math>L</math> as <math>x</math> tends to <math>c</math> . Of course, the way you would do this in general would be to create a function giving you a <math>\delta</math> for every <math>\varepsilon</math> , just as in the example above.
 +
 
 +
===Formal Definition of the Limit at Infinity===
 +
We call <math>L</math> the '''limit''' of <math>f(x)</math> as <math>x</math> approaches <math>\infty</math> if for every number <math>\varepsilon>0</math> there exists a <math>\delta</math> such that
 +
whenever
 +
<math>x>\delta</math>
 +
we have
 +
:<math>\Big|f(x)-L\Big|<\varepsilon</math>
 +
When this holds we write
 +
:<math>\lim_{x\to\infty}f(x)=L</math>
 +
or
 +
:<math>f(x)\to L</math> as <math>x\to\infty</math>
 +
Similarly, we call <math>L</math> the '''limit''' of <math>f(x)</math> as <math>x</math> approaches <math>-\infty</math> if for every number <math>\varepsilon>0</math> , there exists a number <math>\delta</math> such that whenever <math>x<\delta</math> we have
 +
:<math>\Big|f(x)-L\Big|<\varepsilon</math>
 +
When this holds we write
 +
:<math>\lim_{x\to-\infty}f(x)=L</math>
 +
or
 +
:<math>f(x)\to L</math> as <math>x\to-\infty</math>
 +
 
 +
 
 +
Notice the difference in these two definitions. For the limit of <math>f(x)</math> as <math>x</math> approaches <math>\infty</math> we are interested in those <math>x</math> such that <math>x>\delta</math> . For the limit of <math>f(x)</math> as <math>x</math> approaches <math>-\infty</math> we are interested in those <math>x</math> such that <math>x<\delta</math> .
 +
 
 +
==Examples==
 +
Here are some examples of the formal definition.
 +
;Example 1
 +
We know from earlier in the chapter that
 +
:<math>\lim_{x\to8}\frac{x}{4}=2</math>
 +
What is <math>\delta</math> when <math>\varepsilon=0.01</math> for this limit?
 +
 
 +
We start with the desired conclusion and substitute the given values for <math>f(x)</math> and <math>\varepsilon</math> :
 +
:<math>\left|\frac{x}{4}-2\right|<0.01</math>
 +
Then we solve the inequality for <math>x</math> :
 +
:<math>7.96<x<8.04</math>
 +
This is the same as saying
 +
:<math>-0.04<x-8<0.04</math>
 +
(We want the thing in the middle of the inequality to be <math>x-8</math> because that's where we're taking the limit.)
 +
We normally choose the smaller of <math>|-0.04|</math> and <math>0.04</math> for <math>\delta</math>, so <math>\delta=0.04</math> , but any smaller number will also work.
 +
 
 +
;Example 2
 +
What is the limit of <math>f(x)=x+7</math> as <math>x</math> approaches 4?
 +
 
 +
There are two steps to answering such a question; first we must determine the answer — this is where intuition and guessing is useful, as well as the informal definition of a limit — and then we must prove that the answer is right.
 +
 
 +
In this case, 11 is the limit because we know <math>f(x)=x+7</math> is a continuous function whose domain is all real numbers. Thus, we can find the limit by just substituting 4 in for <math>x</math> , so the answer is <math>4+7=11</math> .
 +
 
 +
We're not done, though, because we never proved any of the limit laws rigorously; we just stated them. In fact, we couldn't have proved them, because we didn't have the formal definition of the limit yet, Therefore, in order to be sure that 11 is the right answer, we need to prove that no matter what value of <math>\varepsilon</math> is given to us, we can find a value of <math>\delta</math> such that
 +
:<math>\Big|f(x)-11\Big|<\varepsilon</math>
 +
whenever
 +
:<math>|x-4|<\delta</math>
 +
For this particular problem, letting <math>\delta=\varepsilon</math> works. Now, we have to prove
 +
:<math>\Big|f(x)-11\Big|<\varepsilon</math>
 +
given that
 +
:<math>|x-4|<\delta=\varepsilon</math>
 +
Since <math>|x-4|<\varepsilon</math> , we know
 +
:<math>\Big|f(x)-11\Big|=\Big|x+7-11\Big|=|x-4|<\varepsilon</math>
 +
which is what we wished to prove.
 +
;Example 3
 +
What is the limit of <math>f(x)=x^2</math> as <math>x</math> approaches 4?
 +
 
 +
As before, we use what we learned earlier in this chapter to guess that the limit is <math>4^2=16</math> . Also as before, we pull out of thin air that
 +
:<math>\delta=\sqrt{\varepsilon+16}-4</math>
 +
Note that, since <math>\varepsilon</math> is always positive, so is <math>\delta</math> , as required. Now, we have to prove
 +
:<math>\Big|x^2-16\Big|<\varepsilon</math>
 +
given that
 +
:<math>|x-4|<\delta=\sqrt{\varepsilon+16}-4</math> .
 +
We know that
 +
:<math>|x+4|=\Big|(x-4)+8\Big|\le|x-4|+8<\delta+8</math>
 +
(because of the triangle inequality), so
 +
:<math>\begin{matrix}
 +
\Big|x^2-16\Big|&=&|x-4|\cdot|x+4|\\  \\
 +
\ &<&\delta(\delta+8)\\  \\
 +
\ &<&(\sqrt{16+\varepsilon}-4)(\sqrt{16+\varepsilon}+4) \\  \\
 +
\ &<&(\sqrt{16+\varepsilon})^2-4^2\\  \\
 +
\ &=&\varepsilon+16-16\\ \\
 +
\ &<&\varepsilon\end{matrix}</math>
 +
 
 +
;Example 4
 +
Show that the limit of <math>\sin\left(\tfrac1x\right)</math> as <math>x</math> approaches 0 does not exist.
 +
 
 +
We will proceed by contradiction. Suppose the limit exists; call it <math>L</math> . For simplicity, we'll assume that <math>L\ne1</math> ; the case for <math>L=1</math> is similar. Choose <math>\varepsilon=|1-L|</math> . Then if the limit were <math>L</math> there would be some <math>\delta>0</math> such that <math>\left|\sin\left(\tfrac1x\right)-L\right|<\varepsilon=|1-L|</math> for every <math>x</math> with <math>0<|x|<\delta</math> . But, for every <math>\delta>0</math> , there exists some (possibly very large) <math>n</math> such that <math>0<x_0=\frac{1}{\frac{\pi}{2}+2\pi n}<\delta</math> , but <math>\left|\sin\left(\tfrac{1}{x_0}\right)-L\right|=|1-L|</math> , a contradiction.
 +
 
 +
;Example 5
 +
What is the limit of <math>x\sin\left(\tfrac1x\right)</math> as <math>x</math> approaches 0?
 +
 
 +
By the Squeeze Theorem, we know the answer should be 0. To prove this, we let <math>\delta=\varepsilon</math> . Then for all <math>x</math> , if <math>0<|x|<\delta</math> , then <math>\left|x\sin\left(\tfrac1x\right)-0\right|\le|x|<\varepsilon</math> as required.
 +
 
 +
;Example 6
 +
Suppose that <math>\lim_{x\to a}f(x)=L</math> and <math>\lim_{x\to a}g(x)=M</math> . What is <math>\lim_{x\to a}\big[f(x)+g(x)\big]</math> ?
 +
 
 +
Of course, we know the answer should be <math>L+M</math> , but now we can prove this rigorously. Given some <math>\varepsilon</math> , we know there's a <math>\delta_1</math> such that, for any <math>x</math> with <math>0<|x-a|<\delta_1</math> , <math>\Big|f(x)-L\Big|<\frac{\varepsilon}{2}</math> (since the definition of limit says "for any <math>\varepsilon</math>", so it must be true for <math>\frac{\varepsilon}{2}</math> as well). Similarly, there's a <math>\delta_2</math> such that, for any <math>x</math> with <math>0<|x-a|<\delta_2</math> , <math>\Big|g(x)-M\Big|<\frac{\varepsilon}{2}</math> . We can set <math>\delta</math> to be the lesser of <math>\delta_1</math> and <math>\delta_2</math> . Then, for any <math>x</math> with <math>0<|x-a|<\delta</math> , <math>\bigg|\big(f(x)+g(x)\big)-(L+M)\bigg|\le\Big|f(x)-L\Big|+\Big|g(x)-M\Big|<\frac{\varepsilon}{2}+\frac{\varepsilon}{2}
 +
=\varepsilon</math> , as required.
 +
 
 +
If you like, you can prove the other limit rules too using the new definition. Mathematicians have already done this, which is how we know the rules work. Therefore, when computing a limit from now on, we can go back to just using the rules and still be confident that our limit is correct according to the rigorous definition.
 +
<!-- Can you prove that ln(lim(x->a)f(x))=lim(x->a)ln(f(x))? -->
 +
 
 +
==Formal Definition of a Limit Being Infinity==
 +
 
 +
Let <math>f(x)</math> be a function defined on an open interval <math>D</math> that contains <math>c</math> , except possibly at <math>x=c</math> . Then we say that
 +
:<math>\lim_{x\to c}f(x)=\infty</math>
 +
if, for every <math>\varepsilon</math> , there exists a <math>\delta>0</math> such that for all <math>x\in D</math> with
 +
:<math>0<|x-c|<\delta</math>
 +
we have
 +
:<math>f(x)>\varepsilon</math> .
 +
 
 +
When this holds we write
 +
:<math>\lim_{x\to c}f(x)=\infty</math>
 +
or
 +
:<math>f(x)\to\infty</math> as <math>x\to c</math>
 +
Similarly, we say that
 +
:<math>\lim_{x\to c}f(x)=-\infty</math>
 +
if, for every <math>\varepsilon</math> , there exists a <math>\delta>0</math> such that for all <math>x\in D</math> with
 +
:<math>0<|x-c|<\delta</math>
 +
we have
 +
:<math>f(x)<\varepsilon</math> .
 +
When this holds we write
 +
:<math>\lim_{x\to c}f(x)=-\infty</math>
 +
or
 +
:<math>f(x)\to-\infty</math> as <math>x\to c</math>.
 +
 
 +
==Exact Integrals as Limits of Sums==
 +
Using the definition of an integral, we can evaluate the limit as <math>n</math> goes to infinity. This technique requires a fairly high degree of familiarity with summation identities. This technique is often referred to as evaluation "by definition," and can be used to find definite integrals, as long as the integrands are fairly simple. We start with definition of the integral:
 +
:{|
 +
|<math>\int\limits_a^b f(x)dx</math>
 +
|<math>=\lim_{n\to\infty}\left[\frac{b-a}{n}\sum_{k=1}^n f(x_k^*)\right]</math>
 +
|Then picking <math>x_k^*</math> to be <math>x_k=a+k\frac{b-a}{n}</math> we get,
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{b-a}{n}\sum_{k=1}^n f\big(a+k\tfrac{b-a}{n}\big)\right]</math>
 +
|}
 +
In some simple cases, this expression can be reduced to a real number, which can be interpreted as the area under the curve if <math>f(x)</math> is positive on <math>[a,b]</math> .
 +
 
 +
===Example 1===
 +
Find <math>\int\limits_0^2x^2dx</math> by writing the integral as a limit of Riemann sums.
 +
:{|
 +
|<math>\int\limits_0^2x^2dx</math>
 +
|<math>=\lim_{n\to\infty}\left[\frac{b-a}{n}\sum_{k=1}^n f(x_k^*)\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{2}{n}\sum_{k=1}^n f\big(\tfrac{2k}{n}\big)\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{2}{n}\sum_{k=1}^n\left(\frac{2k}{n}\right)^2\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{2}{n}\sum_{k=1}^n\frac{4k^2}{n^2}\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{8}{n^3}\sum_{k=1}^nk^2\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{8}{n^3}\cdot\frac{n(n+1)(2n+1)}{6}\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{4}{3}\cdot\frac{2n^2+3n+1}{n^2}\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{8}{3}+\frac{4}{n}+\frac{4}{3n^2}\right]</math>
 +
|-
 +
|
 +
|<math>=\frac{8}{3}</math>
 +
|}
  
* [https://mathresearch.utsa.edu/wikiFiles/MAT1214/Limits%20of%20Functions/MAT1214-2.2TheLimitOfAFunctionWS1.pdf The Limit of a Function Worksheet 1]  
+
In other cases, it is even possible to evaluate indefinite integrals using the formal definition. We can define the indefinite integral as follows:
 +
:{|
 +
|<math>\int f(x)dx</math>
 +
|<math>=\int\limits_0^x f(t)dt=\lim_{n\to\infty}\left[\frac{x-0}{n}\sum_{k=1}^n f(t_k^*)\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{x}{n}\sum_{k=1}^n f\big(0+\tfrac{k(x-0)}{n}\big)\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{x}{n}\sum_{k=1}^n f\big(\tfrac{kx}{n}\big)\right]</math>
 +
|}
  
* [https://mathresearch.utsa.edu/wikiFiles/MAT1214/Limits%20of%20Functions/MAT1214_2.2TheLimitOfAFunctionWS2.pdf The Limit of a Function Worksheet 2]
+
===Example 2===
 +
Suppose <math>f(x)=x^2</math> , then we can evaluate the indefinite integral as follows.
 +
:{|
 +
|<math>\int\limits_0^x f(t)dt</math>
 +
|<math>=\lim_{n\to\infty}\left[\frac{x}{n}\sum_{k=1}^n f\big(\tfrac{kx}{n}\big)\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{x}{n}\sum_{k=1}^n\left(\frac{kx}{n}\right)^2\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{x}{n}\sum_{k=1}^n\frac{k^2\cdot x^2}{n^2}\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{x^3}{n^3}\sum_{k=1}^n k^2\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{x^3}{n^3}\sum_{k=1}^n k^2\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{x^3}{n^3}\cdot\frac{n(n+1)(2n+1)}{6}\right]</math>
 +
|-
 +
|
 +
|<math>=\lim_{n\to\infty}\left[\frac{x^3}{n^3}\cdot\frac{2n^3+3n^2+n}{6}\right]</math>
 +
|-
 +
|
 +
|<math>=x^3\cdot\lim_{n\to\infty}\left[\frac{2n^3}{6n^3}+\frac{3n^2}{6n^3}+\frac{n}{6n^3}\right]</math>
 +
|-
 +
|
 +
|<math>=x^3\cdot\lim_{n\to\infty}\left[\frac{1}{3}+\frac{1}{2n}+\frac{1}{6n^2}\right]</math>
 +
|-
 +
|
 +
|<math>=x^3\cdot\left(\frac{1}{3}\right)</math>
 +
|-
 +
|
 +
|<math>=\frac{x^3}{3}</math>
 +
|}
  
  
 +
==Resources==
  
* [https://youtu.be/sdwbppTH0Do The Limit of a Function Part 1] Video Lecture by Instructor Sharon, UTSA.
+
* [https://en.wikibooks.org/wiki/Calculus/Integration_techniques/Infinite_Sums Infinite Sums], Wikibooks: Calculus/Integration techniques
  
* [https://youtu.be/QlxNdVIRxe4 The Limit of a Function Part 2] Video Lecture by Instructor Sharon, UTSA.
+
* [https://en.wikibooks.org/wiki/Calculus/Formal_Definition_of_the_Limit Formal Definition of the Limit], Wikibooks: Calculus
  
* [https://youtu.be/BeyZNXTLels The Limit of a Function Part 3] Video Lecture by Instructor Sharon, UTSA.
+
* [https://mathresearch.utsa.edu/wikiFiles/MAT1214/Limits%20of%20Functions/MAT1214_2.2TheLimitOfAFunctionPwPt.pptx The Limit of a Function] PowerPoint file created by Dr. Sara Shirinkam, UTSA.
  
* [https://youtu.be/HsmId5dqXkQ The Limit of a Function Part 4] Video Lecture by Instructor Sharon, UTSA.
+
* [https://mathresearch.utsa.edu/wikiFiles/MAT1214/Limits%20of%20Functions/MAT1214-2.2TheLimitOfAFunctionNotes1.pdf The Limit of a Function] notes created by Instructor Beatty, UTSA.
  
 +
* [https://mathresearch.utsa.edu/wikiFiles/MAT1214/Limits%20of%20Functions/MAT1214_2.2TheLimitOfAFunctionNotes.pdf The Left and Right Hand Limit] notes created by Instructor Beatty, UTSA.
  
 +
* [https://mathresearch.utsa.edu/wikiFiles/MAT1214/Limits%20of%20Functions/MAT1214-2.2TheLimitOfAFunctionWS1.pdf The Limit of a Function Worksheet 1]
  
* [https://youtu.be/QfPqRMqP5kU Calculus 1 - Introduction to Limits] by The Organic Chemistry Tutor
+
* [https://mathresearch.utsa.edu/wikiFiles/MAT1214/Limits%20of%20Functions/MAT1214_2.2TheLimitOfAFunctionWS2.pdf The Limit of a Function Worksheet 2]
  
 
* [https://youtu.be/HYSI-AHUqRM What is a Limit? Basic Idea of Limits] by patrickJMT
 
* [https://youtu.be/HYSI-AHUqRM What is a Limit? Basic Idea of Limits] by patrickJMT
  
 
* [https://youtu.be/kG_p2vKApOE Lots of Limit Examples, Part 1] by patrickJMT
 
* [https://youtu.be/kG_p2vKApOE Lots of Limit Examples, Part 1] by patrickJMT
 +
 +
==Licensing==
 +
Content obtained and/or adapted from:
 +
* [https://en.wikibooks.org/wiki/Calculus/Integration_techniques/Infinite_Sums Infinite Sums, Wikibooks: Calculus/Integration techniques] under a CC BY-SA license
 +
* [https://en.wikibooks.org/wiki/Calculus/Formal_Definition_of_the_Limit Formal Definition of the Limit, Wikibooks: Calculus] under a CC BY-SA license

Latest revision as of 15:41, 15 January 2022

Whenever a point is within units of , is within units of

In preliminary calculus, the concept of a limit is probably the most difficult one to grasp (after all, it took mathematicians 150 years to arrive at it); it is also the most important and most useful one.

The intuitive definition of a limit is inadequate to prove anything rigorously about it. The problem lies in the vague term "arbitrarily close". We discussed earlier that the meaning of this term is that the closer gets to the specified value, the closer the function must get to the limit, so that however close we want the function to the limit, we can accomplish this by making sufficiently close to our value. We can express this requirement technically as follows:

Formal definition of a limit

Let be a function defined on an open interval that contains , except possibly at . Let be a number. Then we say that

if, for every , there exists a such that for all with

we have

To further explain, earlier we said that "however close we want the function to the limit, we can find a corresponding close to our value." Using our new notation of epsilon () and delta (), we mean that if we want to make within of , the limit, then we know that making within of puts it there.

Again, since this is tricky, let's resume our example from before: , at . To start, let's say we want to be within .01 of the limit. We know by now that the limit should be 4, so we say: for , there is some so that as long as , then .

To show this, we can pick any that is bigger than 0, so long as it works. For example, you might pick , because you are absolutely sure that if is within of 2, then will be within of 4. This works for . But we can't just pick a specific value for , like 0.01, because we said in our definition "for every ." This means that we need to be able to show an infinite number of s, one for each . We can't list an infinite number of s!

Of course, we know of a very good way to do this; we simply create a function, so that for every , it can give us a . In this case, one definition of that works is

So, in general, how do you show that tends to as tends to ? Well imagine somebody gave you a small number (e.g., say ). Then you have to find a and show that whenever we have . Now if that person gave you a smaller (say ) then you would have to find another , but this time with 0.03 replaced by 0.002. If you can do this for any choice of then you have shown that tends to as tends to . Of course, the way you would do this in general would be to create a function giving you a for every , just as in the example above.

Formal Definition of the Limit at Infinity

We call the limit of as approaches if for every number there exists a such that whenever we have

When this holds we write

or

as

Similarly, we call the limit of as approaches if for every number , there exists a number such that whenever we have

When this holds we write

or

as


Notice the difference in these two definitions. For the limit of as approaches we are interested in those such that . For the limit of as approaches we are interested in those such that .

Examples

Here are some examples of the formal definition.

Example 1

We know from earlier in the chapter that

What is when for this limit?

We start with the desired conclusion and substitute the given values for and  :

Then we solve the inequality for  :

This is the same as saying

(We want the thing in the middle of the inequality to be because that's where we're taking the limit.) We normally choose the smaller of and for , so , but any smaller number will also work.

Example 2

What is the limit of as approaches 4?

There are two steps to answering such a question; first we must determine the answer — this is where intuition and guessing is useful, as well as the informal definition of a limit — and then we must prove that the answer is right.

In this case, 11 is the limit because we know is a continuous function whose domain is all real numbers. Thus, we can find the limit by just substituting 4 in for , so the answer is .

We're not done, though, because we never proved any of the limit laws rigorously; we just stated them. In fact, we couldn't have proved them, because we didn't have the formal definition of the limit yet, Therefore, in order to be sure that 11 is the right answer, we need to prove that no matter what value of is given to us, we can find a value of such that

whenever

For this particular problem, letting works. Now, we have to prove

given that

Since , we know

which is what we wished to prove.

Example 3

What is the limit of as approaches 4?

As before, we use what we learned earlier in this chapter to guess that the limit is . Also as before, we pull out of thin air that

Note that, since is always positive, so is , as required. Now, we have to prove

given that

.

We know that

(because of the triangle inequality), so

Example 4

Show that the limit of as approaches 0 does not exist.

We will proceed by contradiction. Suppose the limit exists; call it . For simplicity, we'll assume that  ; the case for is similar. Choose . Then if the limit were there would be some such that for every with . But, for every , there exists some (possibly very large) such that , but , a contradiction.

Example 5

What is the limit of as approaches 0?

By the Squeeze Theorem, we know the answer should be 0. To prove this, we let . Then for all , if , then as required.

Example 6

Suppose that and . What is  ?

Of course, we know the answer should be , but now we can prove this rigorously. Given some , we know there's a such that, for any with , (since the definition of limit says "for any ", so it must be true for as well). Similarly, there's a such that, for any with , . We can set to be the lesser of and . Then, for any with , , as required.

If you like, you can prove the other limit rules too using the new definition. Mathematicians have already done this, which is how we know the rules work. Therefore, when computing a limit from now on, we can go back to just using the rules and still be confident that our limit is correct according to the rigorous definition.

Formal Definition of a Limit Being Infinity

Let be a function defined on an open interval that contains , except possibly at . Then we say that

if, for every , there exists a such that for all with

we have

.

When this holds we write

or

as

Similarly, we say that

if, for every , there exists a such that for all with

we have

.

When this holds we write

or

as .

Exact Integrals as Limits of Sums

Using the definition of an integral, we can evaluate the limit as goes to infinity. This technique requires a fairly high degree of familiarity with summation identities. This technique is often referred to as evaluation "by definition," and can be used to find definite integrals, as long as the integrands are fairly simple. We start with definition of the integral:

Then picking to be we get,

In some simple cases, this expression can be reduced to a real number, which can be interpreted as the area under the curve if is positive on .

Example 1

Find by writing the integral as a limit of Riemann sums.

In other cases, it is even possible to evaluate indefinite integrals using the formal definition. We can define the indefinite integral as follows:

Example 2

Suppose , then we can evaluate the indefinite integral as follows.


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