My question is in two parts; the first is, what is the $|$ operator called? Here's an example of it in use:
$$(x + 5)|_{x=3} = 8$$
My second question is, how do I use this operator for more than one variable? For example, would I write $(x + y)|_{x=3,y=3}$? Or $(x + y)|_{x=3}|_{y=3}$? Or another variation?
Any help is appreciated.
The operator $\mid$ usually reads as "such that" or "given that".
In your example, it works pretty clearly: Take $(x+5)$, given that $x=3$. Then $3+5 = 8$.
As another example, $A = \{x \mid x^2 = 4\}$ is the set of all $x$ such that $x^2 = 4$.
So to respond to your second question, you would write $(x+y)|_{x=3,y=3}$. (Given the informal definition I wrote above, the second variation wouldn't make a whole lot of sense.)
