What does "\" mean in math In a Linear Algebra textbook I am reading, the following is stated: $b\notin \operatorname{span}(A \cup \{a\})\setminus \operatorname{span}(A)$. It does so without explaining what "$\setminus$" means. I apologize if this question does not belong here but I just want to understand what it means. I can close the question if someone just comments on its meaning.

If $A$ and $B$ are two sets, then
$A \setminus B$ means the set that contains all those elements of $A$ that are not in $B$.
Guess this is what you talking about..
