※ 引述《easterday (數牙)》之銘言：
: 問一個笨問題
: 從書上看到這樣的定義
: U is relatively open in E iff there is an open set A,which is subset of U,
: st. U=(A and E)
: 那有哪些實例U,E可以顯示
: U is relatively open in E but U is not open.
: 我好像舉不太出來..
取|R上的interval
E=[0,2]
U=[0,1)
那麼U在|R上不是open
但跟E relatively open
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