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there is a basic lemma in topology, saying that:

Let X be a topological space, and B is a collection of open subsets of X. If every open subset of X satisfies the basis criterion with respect to B (in the sense, that every element x of an open set O is in a basis open set S, contained in O), then B is a basis for the topology of X.

With this lemma at hand, it is asked to show that the collection:

[tex]\mathcal{B}=\{\text{the set of balls}\, B_r(x)\, \text{with rational radius, where x has rational coordinates}\}[/tex] is a basis for the Euclidean topology on R^n.

Take any set O, which is open in the Euclidean metric space sense, i.e. any point in O has a ball of radius [tex]\varepsilon[/tex] around it, which is contained in O.

1. If x has rational coordinates, take a ball, whose radius is a rational number smaller than the given [tex]\varepsilon[/tex].

2. If x has irrational coordinates, we know that the set of points with irrational coordinates is dense in R^n. Hence any such x is between some points with rational coordinates, say a and b. Then it is in the ball around a with radius r=b-a, which is from [tex]\mathcal{B}[/tex] and by 1. contained in O.

Is this explanation correct?

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# Basic lemma in topology

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