# Quick (dumb) Notation Question

1. May 21, 2009

### AOA

I know writing $$\frac{\partial^{1}u}{\partial x^{1}}$$ is "bad" notation, but can I still write the sum:

$$\sum^{n}_{k=1}\frac{\partial^{k}u}{\partial x^{k}}$$

??

THANK YOU

PS. Being able to write LaTeX equations is actually awesome.

2. May 22, 2009

### CompuChip

When you write it explicitly it looks a bit weird, but I see no objection to using it in such a sum.

3. May 22, 2009

### AOA

OK. I just wanted to make sure. Thanks!

4. May 22, 2009

### denisv

Can I ask what kind of situation you're getting that sum in?

5. May 22, 2009

### arildno

When dealing with just the first derivative of some function, it is superfluous, but completely analogical to the case that $x=x^{1}$
Neither of these two notations are "bad" in any way.

6. May 22, 2009

### HallsofIvy

I would even accept
$$\sum_{k=0}^n \frac{\partial^k u}{\partial x^k}$$
with the understanding that
$$\frac{\partial^0 u}{\partial x^0}= u(x)$$