Re: [微積] [1,∞) 是開還是閉？

[armopen (八字-風水-姓名學)]

※ 引述《ntme (ntme)》之銘言：
: 想請問[1,∞)是開集合還是閉集合，or非開非閉
: (-∞,∞)是開集合嗎？
: 一直搞不清楚∞的點...

Let |R = (-∞,∞).

[1,∞) is a closed set in |R since its complement (-∞,1) relative to |R

is an open set in |R.

|R = (-∞,∞) is both an open and a closed set. Think about its complement

relative to |R, leave you as an exercise.

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