# Symbol for partial derivative not used for partial integrals?

1. Mar 5, 2013

### MostlyHarmless

$${\frac{∂(xy)}{∂x}=x}$$ Going backwards. If we took,
$$∫x dy$$ we get $$xy+f(x)$$

Now, the only way that
$$∫x dy$$
is a valid operation, is if we know that we came from a partial derivative. Why, when taking a partial integral, do we not denote like this:
$$∫x ∂y$$
It seems like it would be less confusing.

2. Mar 5, 2013

### micromass

The $d$ in the integral is actually the symbol for the so-called "exterior derivative". You should read up on differential forms, I think that will clear up some confusions. A book like Spivak's "calculus on manifolds" is a nice place to start!

3. Mar 5, 2013

### MostlyHarmless

The first equation was supposed to be with respect to y not x. >.<

But regardless, thanks for the info. I'll look into it when I get some time. I should really be doing homework right now too.

What has my life become that my idea of procrastination is participating in a math forum??

4. Mar 5, 2013

### micromass

A few years ago, I had to study for an exam. I said I was going to check the internet for 5 minutes and I found PF. Now I'm a mentor...

5. Mar 5, 2013

### MostlyHarmless

This IS a great place. Very intellectually stimulating. I could read threads for hours and hours here.

6. Mar 6, 2013

7. Mar 6, 2013

### Curious3141

You're also mental. :tongue:

8. Mar 6, 2013

### WannabeNewton

Ain't that the truth (did you know he threatened to mail me 100s of copies of stewart if I didn't do Reed's Functional Analysis text?)

9. Mar 6, 2013

### Curious3141

So call his bluff and sell them on ebay. PROFIT!!!

10. Mar 6, 2013

### micromass

I would think that is is very immoral to actually demand money for a book like Stewart...

11. Mar 6, 2013

### joeblow

The expression $$\partial x$$ is precisely equal to dx if all other "variables" are kept constant. Thus, in iterated integrals, where you *do* keep all variables constant, $$dx = \partial x.$$

12. Mar 6, 2013

### Fredrik

Staff Emeritus
That's not true. The y after the d lets us know that the function we're dealing with is the map that takes y to x (as opposed to e.g. the map that takes x to x), and also that we are supposed to evaluate the primitive function of this map at y.

Define $g:\mathbb R\to\mathbb R$ by $g(t)=x$ for all $t\in\mathbb R$.

The result of the "integration" would typically be stated as "xy+C, where C is a constant". But this is just a strange way to say that for all real numbers C, the function $G_C:\mathbb R\to\mathbb R$ defined by $G_C(t)=xt+C$ for all $t\in\mathbb R$ is a primitive function of g. If $f:\mathbb R\to\mathbb R$, then f(x) is a real number. So $G_{f(x)}$ is a primitive function of G. That's why you can write
$$\int x dy = G_{f(x)}(y) =xy+f(x).$$

13. Mar 6, 2013

### Fredrik

Staff Emeritus
It's hard to explain this stuff because of how weird the indefinite integral notation really is. Consider e.g. $\int x^2 dx$. What exactly does this notation represent? Is it a number, a function, or a set of functions? People often write things like
$$\int x^2 dx=\frac{x^3}{3}+C$$ and then say something like "...where C is a constant". But there are several strange things going on here. First of all, x is a dummy variable on the left, but not on the right. Second, C is an arbitrary constant.

The second observation suggests that $\int x^2dx$ should really be thought of as a set. The first observation suggests that this is a set of functions, not numbers. So if we want our notation to make sense, we should write something like this instead:
$$\int x^2 dx =\left\{t\mapsto \frac{t^3}{3}+C\,\bigg|\,C\in\mathbb R\right\}.$$ If you don't like the "mapsto" notation ($\mapsto$), then we'd have to write something like this instead:
$$\int x^2 dx =\left\{F_C\,\big|\,C\in\mathbb R\right\},$$ where for each real number C, $F_C:\mathbb R\to\mathbb R$ is defined by $F_C(t)=t^3/3+C$ for all $t\in\mathbb R$.

14. Mar 6, 2013

### MostlyHarmless

If you could take $$∫xdy=xy+c$$ Withoutt knowing we are working backwards from a partial derivative. Then there would be no need to use seperation of variables when solving a differential equation. For example. $$xdy-ydx=0$$ The solution to this is not $$xy-yx=c$$ Which would be 0=c. The solution is $$lny-lnx=c$$ I could be misguided in this, but that is my understanding.

15. Mar 6, 2013

### Fredrik

Staff Emeritus
I think the problem here is that to obtain xy-yx=c, you simply inserted an integral sign in front of each term. Why should the equality still hold after that operation?

I don't see what your example has to do with separation of variables. That phrase usually refers to a rewrite like $\phi(x,y)=X(x)Y(y)$ that gives us two ODE's instead of one PDE.

You wrote $x dy - y dx = 0$. I interpret this as a weird way of writing dy/dx = y/x, which can also be written as f'(x)=f(x)/x. This implies
$$\frac 1 x =\frac{f'(x)}{f(x)} =\frac{d}{dx}\log f(x).$$ Now this is something we can integrate, to get $\log x=\log f(x)+C$, and if we set y=f(x), this is equivalent to your result.

I suppose we can obtain this result more quickly by rewriting the original equation as
$$\frac{dx}{x}=\frac{dy}{y}.$$ Now we actually get the right result if we insert an integral sign in front of both sides. But unless you know a theorem that explains the exact conditions that must be satisfied for this sort of trick to work, you're going to have to treat the result as a guess, and verify the result by other means.

16. Mar 6, 2013

### MostlyHarmless

I was just trying to think of some example of a differential equation, and that is what I came up with.

I'm not sure if we are on the same page. I failed to mention that my calculus background is half way through elementary differential equations, and oversight on my part. When I made the OP I had Differential equations in mind. We have several methods of solving ODE, generally all of these methods attempt to get "only x's in front of dx" and "only y's in front of dy." If we are allowed to take $$∫xdy$$ without the distinction of "xdy" coming from a partial derivative. Then differential equations, would be a bucket of rainbows and lollipops, no? So that is my issue.

To restate it, my understanding is that $$∫xdy$$ is not a valid operation, in the context of differential equations at least, unless you know that xdy came from taking a partial derivative.

17. Mar 7, 2013

### Fredrik

Staff Emeritus
I disagree, for the reasons stated in post #12. It seems to me that you're confusing the validity of $\int x dy$ with the validity of inserting integral signs in front of each term in any equation involving dx and dy separately. The latter operation seems to be valid when each term involves only one variable, but I don't immediately see why.

It's been a long time since I studied differential equations, so I don't remember any of those methods to solve them. If you're half-way through the course, you already know more of them than I do. Separation of variables (the kind I described) and the rewrite $f'(x)/f(x)=d/dx \log f(x)$ are pretty much the only tricks I remember.

Last edited: Mar 7, 2013
18. Mar 7, 2013

### pwsnafu

Can you come up with an example where you don't know the number of dependent variables?

19. Mar 7, 2013

### Fredrik

Staff Emeritus
We only have to think about dependent/independent variables when the notation is bad to begin with, e.g. when y is a function and someone has written y instead of y(x) out of laziness. If we assume that the notation contains all the information we need, then expressions like $\int x dy$ are unambiguous.

20. Mar 7, 2013

### pwsnafu

That's side stepping my point. This thread is framed in the context of DEs. You always know what your variables are when you set up your DE in the first place. That's why I'm asking the OP for an example where variables are unknown. I can't envision a case where OP's problem crops up.