Re: [高微] 拓撲

[mack (腦海裡依然記得妳)]

※ 引述《y15973 (B.H.Justin)》之銘言：
: 一題
:          n
: 若A，B為R 且A、B皆closed
: 令A+B={ a+b | a屬於A，b屬於B }
: 試舉一例A+B亦為closed
A = {(0,0,0,...,0)} is closed
B = {(0,0,0,...,1)} is closed
A+B = {(0,0,0,...,1)} is closed

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推 herstein    :XD
推 zako1113    :隨便想都是, 倒是想問有沒有A+B not closed的例子?
推 znmkhxrw    :樓上那個, A={-n：n€N} , B ={n+1/n：n€N}
推 herstein    :二樓很強喔XD
推 powerkshs   :的確隨便想都是啊,猜元po題目記錯XD
推 zako1113    :感謝三樓很強