How to read $A=[0,1]\times[a,5]$ I have this problem: consider the two sets $A$ and $B$
$$A=[0,1]\times [a,5]$$ and $$B=\{(x,y):x^2+y^2<1\}$$
What are the values of $a$ that guarantee the existence of a hyperplane that separates $A$ from $B$.
Given a chosen value of $a$, find one of those hyperplanes.
My main problem is axiomatics: how do I read: $A=[0,1]\times[a,5]$, what's with the $\times$?
Thank you

The $\times$ stands for cartesian product, i.e. $X\times Y=\{(x,y)\mid x\in X, y\in Y\}$. 
Whether ordered pairs $(x,y)$ are considered a basic notion or are themselves defined (e.g. as Kurtowsky pairs) usually does not matter. See alo here. 
