[分析] 有關於Relatively open of R^n

[easterday (數牙)]

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..


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