[分析] closed set
A and B are nonempty closed subset in R.
Define A+B = {a+b | a in A, b in B}
A*B = { ab | a in A, b in B}.
Prove or disprove A+B, A*B are closed.
沒辦法馬上想出反例
因此剛剛決定試著證明命題為真
從E=cl(E) 的方式出發 但發現這樣並不好做
暫時想不出其他好方法 懇請高手板友們指教
感恩!!
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