Exercise 4.11. For each of the following, state whether it is true or false. If true, prove. If false, provide a counterexample.
(i) Let X and Y be sets from Rn. If X ⊂ Y then X is closed if and only if Y is closed.
(ii) Let X and Y be sets from Rn. If X ∩Y is closed and convex then either X or Y is closed and convex (one or the other).
(iii) Let X be an open set and Y ⊆ X. If Y ≠ ∅, then Y is a convex set.
(iv) Suppose X is an open set and Y is a convex set. If X ∩ Y ⊂ X then X ∪ Y is open.
(v) If A and B are closed sets, and A ∩ B = ∅ then A ∪ B is a closed set.
(vi) If a set A is closed, then A cannot be open.
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