推 goodGG :想一下 兩個開集交集為開集 怎麼證吧 ^^ 09/30 23:46
→ zako1113 :the union's complement is a finite intersection 09/30 23:48
→ zako1113 :of open sets 09/30 23:48
推 jacky7987 :會證兩個,然後用induction就可以了 10/01 00:34
推 bineapple :直接用closed的定義證就好了 10/01 08:35