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Homework Statement
I am having somewhat a difficult time just understanding a simple concept. I am trying to prove that every open subset G of a separable metric space X is the union of a sub collection {Vi} such that for all x belongs to G, x belongs to some Vi (subset of G).
I am thinking of this problem for the sake of simplicity in just R. Let (a,b) be any open set G in the R. Then I need to show that (a,b) can be shown as union of countable collection of neighborhood of Q ( set of rational numbers).
Homework Equations
The Attempt at a Solution
I need to show that (a,b) is a union of countable collection of {Vis} where each Vi is a neighborhood (qi-e, qi+e) , where qi belongs to Q. Let 's say 0<a <b
I take any point d in Q, such that d belongs to (a,b). This is possible because rational numbers are dense in (a,b). Now, I have some Vi = (d-ri, d+ri) for some b-d>ri >0 and ri belongs to Q. In other words, Vi is within (a,b). Now, I can keep on finding another rational number rj,rk, until d+rj > b and d-rk <a. Let's say d+ rj = b + h' and d-rk = a-h (where h,h' >0).
But, d +rj belongs to Vj and d - rk belongs to Vk. And so (a,b) can be represented as
Vi U ... Vk U Vj. This is countable.
Is my solution ok? Is there an elegant way of proving the same?
Please help. I am stuck. Because I need to extend this to any metric space X (not necessarily R). Thanks