→ tiwsjia :17540 篇的推文有給反例。Y 01/17 13:54
→ KevinJames :可是你給的B是open的阿 01/17 16:00
→ KevinJames :應該說B不是closed,當n->∞,1不在B集合裡 01/17 16:10
→ KevinJames :我看懂了,一直看成(n+1)/n 01/17 19:35
※ 引述《IminXD (Encore LaLa)》之銘言：
: 題目是: Let A,B ≦ R^n be closed set.
: Does A+B = {x+y | x€A and y€B} have to be closed?
: 解答是找例子 A={(x,0) €R^2 | x€R}
: B={(t,1/t) €R^2 | t>0}
: 我自己是從定義著手
: A is closed => (R^n - A) is open
: B is closed => (R^n - B) is open
: to defined whether A+B is closed or not,we consider R^n-(A+B)
: R^n-(A+B) = R^n-A-B = (R^n-A)-B
: since R^n-A is open , and B is close
: => R^n-(A+B) is not open
: Hence A+B is not closed ##
: 問題點就是說 open - closed = closed 能不能推過去..
: 感覺這個也是要舉例證明.....囧
關於R^n-(A+B) = R^n-A-B = (R^n-A)-B
其中R^n-(A+B),其中'-',應該是'\'也就是A+B餘集,且'+'並非聯集
所以這樣的作法是不行的,從例子下手會較好
另外,我對於當n=1時,不知該找什麼反例
想問板上是否有人可替小弟解答??
--
※ 發信站: 批踢踢實業坊(ptt.cc)
◆ From: 140.127.223.156