[分析] 稠密性

[happyhello (衝阿 火箭!!)]

If A is dense in R^n and U is open , prove that A∩U is dense in U.

Is this true if U is not open?

無法體會U是不是open有什麼影響＝ ＝

懇請高人指點 <(__ __)>

感激不盡

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推 LimSinE:不然，取A=Q，U=R\Q in R.