Suppose I have $f:\mathbb{R}^2 \to \mathbb{R}$. What conditions do I need to say that
$\lim_{x \to a} \lim_{y \to b} f(x,y) = \lim_{y \to b} \lim_{x \to a} f(x,y)$
?
What about in a more general case, by taking $X,Y$ and $Z$ topological (Hausdorff) spaces and $f$ from $X \times Y$ to $Z$ ?
Thank you