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I need to prove that if $A$ and $B$ are two closed sets in $\mathbb R^n$, then $A + B$ is an $F_\sigma$.

I started by writing out $A$ and $B$ each as an $F_\sigma$ by intersecting each with balls of center $0$ radius $n$, and taking the union, I don't know if that's a good idea.

Any hint is appreciated. Thanks.

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    It is interesting to mention that this exercise implies that the sum of two $F_{\sigma}$ sets remains and $F_{\sigma}$ set.2017-01-05

1 Answers 1

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If $A$ is closed and $B$ is compact, show that $A+B$ is closed. (Hint: Use sequential compactness.) In general, write $B$ as a countable union of compact sets. If you need more detail, please let me know and I will gladly provide it.