Explains it nicely. I can not improve on it.
In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.
Yep, another nice page.
It might be worth mentioning that an "outward facing" square bracket is sometimes used instead of a round one for open intervals.
eg. (0,1) = ]0,1[
() is my preferred notation though.
Why did the vector cross the road?
It wanted to be normal.