In: Advanced Math
1.2.2*. Find an example to show that the phrase "finitely many" is necessary in the statement of Lemma 1.2.3 (iii).
Lemma 1.2.3.
(i) Rn and empty set are open in Rn
(ii) The union ofo pen subset of Rn is open.
(iii) The intersect of finitely many open subsets Rn is open.