# hobz

Senior Members

243

1. ## Proof of smooth functions

Suppose I have f(x)=e^x, how can it be proven that it is smooth?
2. ## Units and multiplication

Ah yes. Of course. Thank you both.
3. ## Units and multiplication

I don't think I have made my self clear. The problem is that if multiplication is repeated addition, how do I add the units to make them square?
4. ## Units and multiplication

To my knowledge: $3 \cdot 5 = 5 + 5 + 5$ However $3 \, \mathrm{m} \cdot 5 \,\mathrm{m} = 15 \, \mathrm{m}^2 \neq 5 \,\mathrm{m} + 5 \,\mathrm{m} + 5 \,\mathrm{m}$ What is wrong?
5. ## Why cant fish breathe air?

In practice if you keep the gills wet, the fish will be able to survive above water, breathing air.
6. ## The derivative as a ratio

Surely an injective function can still has a derivative? You mean as a ratio?
7. ## The derivative as a ratio

I see. So with ordinary "high school" mathematics, the derivative as a ratio holds just fine. Which also would explain how the founding fathers (before the derivative as a limit) were able to use and deduce stuff.
8. ## The derivative as a ratio

Concerning differentials, when is 1/dy/dx not equal to dx/dy? When can the derivative not be thought of as a ratio?
9. ## Statement vs. Hypothesis

Excellent! Is there a different meaning to hypothesis in maths? Merged post follows: Consecutive posts mergedWikipedia http://en.wikipedia.org/wiki/Theorem equates a theorem to a statement proven true and says that the theorem consists of hypotheses and a conclusion which is a necessary consequence of the hypothesis. Merged post follows: Consecutive posts mergedSuggesting that the definitions are different depending on which topic they are used in.
10. ## Statement vs. Hypothesis

I see. I had the impression that a hypothesis was more like "P" and a theorem was "If P then Q". What is a theorem? A side note. A statement is sort of a binary axiom?
11. ## Statement vs. Hypothesis

What is the difference between a statement and a hypothesis?
12. ## I have a new Idea ...

I don't think you can do it simpler than for i=0:N for j=0:M ... which is the general method for iterating through a matrix.
13. ## Differentials in the product rule

I will try to dig deeper using the links you posted. Thanks for a good discussion. I appreciate it.
14. ## Differentials in the product rule

So is $\frac{\partial}{\partial x}$ more "correct" than $\frac{d}{dx}$ which is more of a special case?
15. ## Differentials in the product rule

The notation looks advanced. I will have to look it up somewhere. Is it fair to say, that in Leibniz' days, the $\frac{dy}{dx}$ was thought of as a ratio, until the more rigid approach with limits gave a definition where nilpotents and arguments as to $(dx)^2$ could be discarded while $dx$ could not, were not needed? The notation $\frac{dy}{dx}$ is a legacy that reminds us of the origin (a ratio) but really isn't. If division is defined, then $dy$ and $dx$ can be split up into each of their own limits and be multiplied on both sides etc. rendering algebraic operations valid in algebra that has +,-,*,/ defined (as I suppose was the case around Leibniz days). If division is undefined, the notion of $\frac{dy}{dx}$ doesn't change; it is still the derivative or how much $dy$ changes when $dx$ changes, but it is no longer a ratio, and the derivative has to be defined in another way. I mean, when introduced in high school, the classic secant to tangent method is introduced, and the limit is called the derivative. While I always thought that this presented the idea of the derivative in any space, it really presents a way of calculating the derivative in that particular space where +,-,*,/ are defined. If we move into a vector field, the idea of measuring how the vector field changes is still perfectly valid. But to calculate the change is no longer a ratio issue, but the term $\frac{dy}{dx}$ lingers on, and becomes an operator in its entirety. $dx$ looses it meaning, the only thing that matters is the operator: $\frac{dy}{dx}$. Can you verify or completely obliterate my thoughts the past days?
16. ## Differentials in the product rule

I mean, when the notion of division is not defined. For instance in a vector space as ajb suggested. I think we arrived at the special notation $\frac{dy}{dx}$ is not a ratio, but defined in terms of limits of a ratio (the normal derivative definition). However, I have seen plenty of derivations of formulas in physics, where dy is treated as a "thing" that easily can be divided with dx. Or multiplying $\frac{dy}{dx} = f(x)$ by dx and integrate both sides is also frequently used.
17. ## I have a new Idea ...

I am having an extremely difficult time trying to understand this post. I think you should reformulate your idea.
18. ## I know this is crazy, but what if ...

Take a look at this using resonant energy transfer.I has little to do with guns though..
19. ## Multiplayer games and the internet

They will probably start moving right away. They will most certainly not move on the host machine before the move order is received with a 2 sec delay. The game could also wait for the host to confirm that your troops have received new orders, and not allow the move order to be executed before the host has confirmed. In this case, there would be a 4 sec delay. But by that time you would probably have quit the game and found something else to do.
20. ## Numerical Input to Computer

You'd be far better of defining pi as a constant at compile time. First of all you can get exact precision to the degree that 'double' (or long double if you prefer) offers. Second, calculating a constant value every time the function is called, is just a waste of power. (Hopefully the compiler will fix this for you.)
21. ## Differentials in the product rule

So how would you go about defining differentiation where the is no division?
22. ## Differentials in the product rule

I assume that by changing x I change y since y=f(x). So by changing x by dx, I change y by dy. My goal is then to find out how much the change in y (dy) is in response to a change in x (dx). Merged post follows: Consecutive posts merged I will attempt this a quiet evening. If I understand your notation correctly, then $\frac{\partial y}{\partial x}$ means the derivative. This a notation, and not a division; the notation cannot be separated into a $\partial y$ and $\partial x$. It serves as an operator. If so, then this operator relates the change in x $dx$ to the change in y $dy$. Am I off? It makes sense in systems where division is not defined that $\frac{\partial y}{\partial x}$ is NOT a division. Is the notation $\frac{\partial y}{\partial x}$ still related to Taylor's theorem?
23. ## Differentials in the product rule

Thanks for the reply. I don't understand this. Failure of a derivative to be a derivation? I am unfamiliar with this notation. Does it mean that $\gamma$ maps x to y? (Can't figure out $\gamma^{*}y=y(x)$.) That is to say that $dx$ is a differential where $\partial x$ is not? I thought they were identical execept the partials denoted a function of several variables being differentiated with respect to one while treating the other as constant. Does that mean that the derivative is not dependent on division to exist in a given context in order to work? (This would mean that the notation is sort of abused). I cases where division is defined (a*b=c <=> b=c/a i guess) is it then problematic to divide? Is this different from $dy = dx\frac{dy}{dx}$ when y is only dependent on x? (Related to a previous encounter http://www.scienceforums.net/forum/showthread.php?t=44865)
24. ## Differentials in the product rule

But surely there are no terms in the Taylor expansion that are not there when taking the derivative. I mean, the Taylor expansion must yield zero for the second term, right? If the Taylor expansion is used on $y=u(x)\cdot v(x)$ would the term $\frac{dudv}{dx}$ also appear? On another note. Is the differential notation not perfectly valid and equivalent to the difference quotient notation? If $\frac{dy}{dx} = \lim_{\Delta x \rightarrow 0} \frac{f(x+\Delta x)-f(x)}{\Delta x}$ then what is $\frac{dy}{dx}$ in the differential notation? If: $y = x^2$ Changing x by the amount dx gives the change dy in y: $y + dy = (x + dx)^2$ $y + dy = x^2 + 2x\cdot dx + (dx)^2$ $dy = 2x\cdot dx + (dx)^2$ $\frac{dy}{dx} = 2x + dx$ Suggesting that the rate of change in y with respect to x also depends on dx. Now you have to argue that dx is negligible, which is handled by limits in the difference quotient way of writing this. So is the differential notation inferior?
25. ## Differentials in the product rule

What I write looks identical to both http://en.wikipedia.org/wiki/Product_rule#Discovery_by_Leibniz and http://en.wikipedia.org/wiki/Product_rule#Using_non-standard_analysis. In both cases the dudv is discarded. But what happens if I keep it?
×

• #### Activity

×
• Create New...

## Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.