+1 vote
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Let $U_{1}\supset U_{2} \supset...$ be a decreasing sequence of open sets in Euclidean $3$-space $\mathbb{R}^{3}$. What can we say about the set $\cap U_{i}$ ?

1. It is infinite.
2. It is open.
3. It is non-empty.
4. None of the above.
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+1 vote