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**rgriss1** Given the metric space (M,d) where M = **R**x**R** and d(x,y) = the Euclidean metric, what is the interior of the subset of M, **Q**x**Q**?

I realize it is the empty set because there are points that lie in the open ball of any element of **Q**x**Q **that are not actually in the set **Q**x**Q**.

What I am struggling with is giving a formal, reasonable justification of why the interior is the empty set.

Any help or suggestions to get me thinking in a good direction would be greatly appreciated. Thank you so much, in advance!