Is this Space Locally Compact?

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SUMMARY

The space X = ℝ \ {a + b√2 : a, b ∈ ℚ} is not locally compact. The discussion references the Baire theorem, which states that in a locally compact Hausdorff space, the intersection of dense open sets is dense. While one participant initially questioned this, another clarified that the intersection of dense subsets is indeed dense in this context, drawing parallels to ℝ \ ℚ, which is completely metrizable. The key to proving non-local compactness lies in examining the canonical embedding i: X → ℝ and the properties of compact subsets.

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Homework Statement




Is the space [tex]X=\mathbb{R\backslash}\left\{ a+b\sqrt{2}\::\: a,b\in\mathbb{Q}\right\}[/tex] locally compact?

Homework Equations


According to the baire theorem, in a locally compact hausdorf space, the intersection of dense open sets is dense.

The Attempt at a Solution


I'm leaning for a no here and want to show a violation of the baire theorem. I'm not sure how to go about constucting my dense sets because I am not sure what is dense in this subset.
Thanks
Tal
 
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Hmm, nice try. But I think that the intersection of dense subsets IS dense in this space. Because this space looks a lot like [tex]\mathbb{R}\setminus \mathbb{Q}[/tex], which is completely metrizable, and hence the Baire theorem holds in this case.

Nevertheless, I think the space is not locally compact, but you'll have to show it directly.
Here's a hint. Consider the canonical embedding [tex]i:X\rightarrow \mathbb{R}[/tex]. If K is a compact subset of X, then i(K) is compact in [tex]\mathbb{R}[/tex]. This gives you a good characterization of compact sets...
 

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