# Qm notation

1. Aug 31, 2012

### g.lemaitre

deleted

2. Aug 31, 2012

### Muphrid

Do you know what the brackets mean in general?

Edit: pay attention to this line in particular.

This tells you very explicitly what the interpretation of both quantities should be.

3. Aug 31, 2012

### g.lemaitre

I didn't see that sentence the square of the average is not the same as the average of squares. But now i do.

4. Aug 31, 2012

### g.lemaitre

Nevertheless I'm truly amazed that you saw the question almost the moment I posted it and answered it in about 5 seconds. Wow!

5. Aug 31, 2012

### Staff: Mentor

I think the <> notation just means average.

so the <j^2> means the average of the squares of j values

and <j>^2 is the square of the average of j values

6. Aug 31, 2012

### g.lemaitre

But I guess what the brackets mean is that you have the take the average of what is between them, so pretend { is a bracket. If you know the latex for brackets please let me know.

{j^2} where j is 2,3,4 would be the average of 4 9 16 hence a little above 9 whereas {j}^2 would be 9 exactly, right?

7. Aug 31, 2012

### Muphrid

Use \langle and \rangle for pretty brackets (not horrendously bad ones, which are what <> give you).

Otherwise, yes, you have the basic idea now. You should be accustomed to seeing $\langle j^2 \rangle - \langle j \rangle^2 = \sigma_j^2$ as well. This is one formula for the variance.