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Since differential quantities are defined as any variable /function tending to zero ($\lim_{x\to0} x= dx$). This is basically the smallest value that we can imagine. Doesn't this mean that there is only one smallest value that ew can imagine? Doesn't the existence of another differential quantity, say $dl$ also mean that $dl$ can be smaller than dx? For example,right angled triangle

Means that $dl>dh$ ? Or do the differential quantities differ by a differential differential amount that is negligible in first order integrations?

EDIT: It seems I was mistaken in my understanding of limits and differentials. I am not taking down this question as it may be useful to others with the same confusion.

Skawang
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  • Although two quantities may both be tending to zero, one of them may be tending to zero faster than the other: consider the difference between $x$ and $x^2$ as $x \to 0$. – Théophile Nov 18 '15 at 04:25
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    That is not the definition of a differential. – Taylor Nov 18 '15 at 04:52
  • Take a look at this video (Powers of 10, 1977) https://www.youtube.com/watch?v=0fKBhvDjuy0 . Consider the relative size of Jupiter's orbit compared to that of Earth's when viewed from various distances. No matter how far away we get from our solar system, Jupiter's apparent orbit is always larger than that of earth's. – John Joy Nov 18 '15 at 22:38

2 Answers2

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There seems to be a(t least one) serious (but rather common) misunderstanding.

When you say things like "the smallest value we can imagine," what do you mean? A natural answer (assuming that, by "smallest," you mean "closest to $0$") is $0$. However, context leads me to believe that you are referring to some nonzero value. Now, here's the problem: if $v$ is a non-zero value, then so is $\frac12v,$ and the latter is strictly smaller! Hence, there is no such thing as "the smallest nonzero value we can imagine."


Added: It seems you're also misunderstanding what a limit is. Again, this is a very common mistake to make! Within the last two weeks, I was explaining this to another user. Unfortunately, the question (and my answer to it) have since been deleted, so I can't link to it. Alas!

Let me first give you a few verbal definitions. After each verbal definition, I will give the common rigorous definition and connect the two.

We will be supposing hereinafter that $f$ is a function from $E$ to $\Bbb R$ for some subset $E$ of $\Bbb R.$

We will also be assuming that $x_0$ is a limit point of $E$ in $\Bbb R.$ Put into words, this means that $x_0$ is a point of $\Bbb R,$ and no matter how close we get to $x_0,$ we can always find a point of $E$ that is closer to (and yet distinct from) $x_0.$ Put rigorously: given any $\delta>0,$ there is some $x\in E$ such that $0<|x-x_0|<\delta.$ Here, $\delta$ tells us how "close" we need to get to $x_0$ and $|x-x_0|$ is the distance from $x$ to $x_0.$ So, $|x-x_0|<\delta$ says that $x$ is "even closer" than our required distance $\delta,$ while $0<|x-x_0|$ tells us that $x$ and $x_0$ are distinct. Note that $x_0$ may or may not be an element of $E.$ For example, $0$ is a limit point of the set of non-zero real numbers (of which it is not an element) and of the set of nonnegative real numbers (of which it is an element).

Now, we say that $L$ is a* limit of $f(x)$ as $x$ approaches $x_0$ if we can make sure that $f(x)$ is as close as we like to $L,$ simply by making sure that $x$ sufficiently close to (but not equal to) $x_0.$ Put rigorously: given any $\epsilon>0,$ there is some $\delta>0$ such that for any $x$ in $E$ with $0<|x-x_0|<\delta,$ we have that $|f(x)-L|<\epsilon.$ Here, $\epsilon$ tells us the desired "closeness" level between $f(x)$ and $L,$ so $|f(x)-L|<\epsilon$ says that $f(x)$ is as close to $L$ as we want it to be. Again, $0<|x-x_0|$ tells us that $x$ and $x_0$ are distinct,. Also, $\delta$ shows us the sufficient "closeness" level between $x$ and $x_0,$ so that $|x-x_0|<\delta$. shows that $x$ is sufficiently close to $x_0$ for our purposes. Depending on how close we want $f(x)$ and $L$ to be (that is, depending on how small $\epsilon$ is), we may need to change our requirement for how close $x$ and $x_0$ must be (that is, our $\delta$ may change).

*It turns out that, if there is a limit of $f(x)$ as $x$ approaches $x_0,$ then it is unique, and we denote it by $\lim\limits_{x\to x_0}f(x).$

So, what's the point? Well, let's consider what $\lim\limits_{x\to 0}x$ means, if anything.

The function $f(x):=x$ is readily defined on all of $\Bbb R$--that is, we have $E=\Bbb R$ in this case--and $x_0=0$ is readily a limit point of $\Bbb R.$ We can prove this by adapting the argument from my initial answer (before the "Added" part).

Now, for we want to know if there is some $L$ such that, given any $\epsilon>0,$ we can pick $\delta>0$ such that if $0<|x-x_0|<\delta,$ then $|f(x)-L|<\epsilon.$ Translating this into our particular situation, we want to know if there is some $L$ such that, given any $\epsilon>0,$ we can pick $\delta>0$ such that if $0<|x-0|<\delta,$ then $|x-L|<\epsilon.$ Looking at it that way, it should be clear that if we put $L=0$ and $\delta=\epsilon,$ then this works out just fine. The upshot is this: $$\lim\limits_{x\to 0}x=0.$$

This may come as a shock. After all, didn't we require $x$ to stay away from $0$ in our definition of limit? Well, yes, in part. We required our domain values to be distinct from $0$. Consequently, our range values were also required to be distinct from $0.$ However, the limit was not restricted! Not only does $0$ fit the definition of the desired limit, it is the only number that does!


Added: Let me go through a proof of limit uniqueness in a less formal way that will hopefully be easier to understand.

Claim: If $L$ is a limit of $f(x)$ as $x$ approaches $x_0,$ then it is the only number that is. Hence, $\lim\limits_{x\to x_0}f(x)$ is well-defined.

Heuristic Proof: Take $L'$ to be any number not equal to $L,$ so that the distance between $L$ and $L'$ is positive. Call this distance $d,$ and note that $\frac12d$ is positive. Hence, we have by definition of limit that, for $x$ sufficiently close (but not equal to) $x_0,$ $f(x)$ is within $\frac12d$ of $L.$ Consequently, for such $x,$ the distance from $f(x)$ to $L$ is less than half of the distance from $L'$ to $L.$ Hence, for such $x,$ we have that $f(x)$ is more than $\frac12d$ away from $L',$ and less than $\frac32d$ away from $L'$ (though the latter isn't important). In order to have $L'$ as a limit of $f(x)$ as $x$ approaches $x_0,$ though, we would need to be able to make $f(x)$ be less than $\frac12d$ away from $L'.$ Having showed this to be impossible, we find that for any $L'\ne L,$ $L'$ is not a limit of $f(x)$ as $x$ approaches $x_0.$

The approach above is basically a more general (and direct) version of the formalistic and particular proof I gave in the comments. There, I used proof by contradiction, and "$L$" there corresponds to "$L'$" here. From this claim, it follows that $\lim\limits_{x\to 0}x=0,$ in particular.

Cameron Buie
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  • I have expanded my answer to address your understanding of "limit." Hopefully, it should help you. Please let me know if you have any further questions or confusion about my answer. – Cameron Buie Nov 19 '15 at 05:24
  • could you explain why 0 is the only value that satisgies the limit? It seems that for any $\delta<\epsilon$, $|x-L|<\epsilon$ is true if $|x|<\delta$. Sorry if I am being overly stupid as the reason you didn't mention it must be because it is obvious. Other than that, the answer is clear and the proof is pretty amazing. – Skawang Nov 19 '15 at 17:54
  • Well, suppose that $L$ is some non-zero number that satisfies the limit definition. In other words, for any $\epsilon>0$ we can find a $\delta>0$ such that $|x-L|<\epsilon$ if $0<|x|<\delta.$ In particular, since $L\ne0,$ then $|L|$ is positive, and so $\epsilon_0:=\frac12|L|$ is also positive. Now, note that the statement "$|x-L|<\epsilon_0$" is equivalent to the statement "$-\epsilon_0<x-L<\epsilon_0$," which is in turn equivalent to "$L-\epsilon_0<x<L+\epsilon_0.$" Recalling our definition of $\epsilon_0,$ we see that this is equivalent to $$L-\frac12|L|<x<L+\frac12|L|.$$ (cont.) – Cameron Buie Nov 19 '15 at 18:16
  • If $L$ is positive, then this becomes $$\frac12|L|<x<\frac32|L|,$$ and if $L$ is negative, it becomes $$-\frac32|L|<x<-\frac12|L|.$$ Hence, regardless of wheher $L$ is positive or negative, we can rewrite this as $$\frac12|L|<|x|<\frac32|L|.\tag{$\star$}$$ (cont.) – Cameron Buie Nov 19 '15 at 18:22
  • On the other hand, we assumed that $L$ satisfied the limit definition, so since $\epsilon_0$ is positive, then there is some $\delta>0$ such that $|x-L|<\epsilon_0$ if $0<|x|<\delta.$ Put $$x_0=\min\left(\frac12\delta,\epsilon_0\right).$$ It should be clear that $x_0$ is positive and $0<|x_0|<\delta,$ so by assumption, we have $|x-L|<\epsilon_0.$ However, by $(\star),$ this means $$\epsilon_0:=\frac12|L|<|x_0|=x_0,$$ which is impossible, since $x_0\le\epsilon_0$ by definition! Hence, since our assumption that $L\ne0$ led to an impossibility, then $0$ is the only possible limit. – Cameron Buie Nov 19 '15 at 18:33
  • I suppose you've answered more than is normally required so I've accepted the answer. But if possible, please explain how you took $x_{0}=min(\frac{1}{2}\delta,\epsilon_{0})$. I thought the value of $x_{0}$ was initially taken as 0 in this case. – Skawang Nov 20 '15 at 08:21
  • Oh, my goodness! I had forgotten I did that. To fix it, put $$x_1:=\min\left(\frac12\delta,\epsilon_0\right),$$ and then change all the instances of "$x_0$" that follow to "$x_1$." – Cameron Buie Nov 20 '15 at 14:23
  • Also, if I haven't answered your question, yet, don't feel compelled to accept. Please let me know if there is still some confusion! – Cameron Buie Nov 20 '15 at 14:32
  • In the first part of this answer, I discuss "differential quantities" a bit, and how they don't really mean anything on their own, but only in pairs (or with an integral sign). – Cameron Buie Nov 20 '15 at 14:38
  • $"Put x_{0}=min(\frac{1}{2}\delta, \epsilon_{0})"$ I lost you there. If I am right, you are assigning the value of the minimum of $1/2\delta$ and $\epsilon_{0}$ to $x_{1}$. Why is that? I didn't follow anything after that. I suppose I am not familiar with these kind of rigourous proofs which might be why I have difficulty understanding yours. Besides, what they taught us in class about limits and all is nothing like this; just a vague and intuitive idea. – Skawang Nov 21 '15 at 17:56
  • You are correct about what I was doing. After I came up with $(\star),$ I noticed in particular that $\epsilon_0:=\frac12|L|<|x|$ when $x$ is within $\epsilon_0$ of $L.$ Put another way, $x$ is more than $\epsilon_0$ from $0,$ so forcing $x$ closer to $0$ would not cause $x$ to be as close as we like to $L.$ At that point, I saw the contradiction, but I still had to point it out. So, I used the assumption that $L$ satisfied the limit definition to get a $\delta$ that was supposed to do the trick. (cont'd) – Cameron Buie Nov 22 '15 at 18:41
  • To show the contradiction, then, I needed to show that it didn't do the trick. To make sure that $x_1$ wasn't as close as we liked to $L,$ I needed it to be within $\epsilon_0$ of $0.$ To make sure that $x_1$ was supposed to be as close as we liked to $L,$ I needed it to be within $\delta$ of $0.$ That's why I defined $x_1$ the way I did. I have added a more general (and hopefully, easier to understand) proof of limit uniqueness to my answer. Let me know if you have questions about it. – Cameron Buie Nov 22 '15 at 18:44
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A differential is not a quantity, it is not "tending to zero" and the equation $\lim_{x\to0} x= dx$ is meaningless (by the way, $\lim_{x\to0} x= 0$). An expression like $dl>dh$ also has no meaning.