$X = (\{1\}\cup\{2,3\})\cap(0,4)$
I think the solution is an empty set but I would like confirmation or refutation from you.
I think it is an empty set because we take the union of 2 sets containing just those elements: $1,2,3$ and then we search for the intersection with the open interval $(0,4)$ - which means that 0 and 4 are not included. Thus, the 2 intervals share $1,2,3$ ... now, I feel even more confused but I keep thinking that the solution is an empty set. Am I wrong?