Every open subset of R^p is the union of countable collection of

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In summary, the conversation discusses the fact that every open subset of Rp can be represented as a union of a countable collection of closed sets. The speaker is seeking guidance on how to proceed with this concept, specifically in regards to irrational numbers and their relation to rational numbers within a certain open ball. It is noted that this concept is equivalent to R^p being 2nd-countable.
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vish_maths
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Every open sub set of Rp is the union of countable collection of closed sets.

I am attaching my attempt as an image file. Please guide me on how I should move ahead. Thank you very much for your help.
 

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Take an irrational number ##a## in ##G##. You can find a certain open ball ##B(a,\varepsilon)## which remains in ##G##. What can you tell about the rational numbers in that ball? In particular, if ##q\in B(a,\varepsilon)## is rational, what can you tell about the closed set in the hint?
 
  • #3
This is equivalent to R^p being 2nd-countable.
 
  • #4
I think I got it. Thank you for your comments
 

Related to Every open subset of R^p is the union of countable collection of

What does it mean for a subset to be "open" in R^p?

In mathematics, an open subset of a topological space is a subset that does not contain any of its boundary points. In other words, every point in an open subset has a small neighborhood completely contained within the subset.

What is R^p?

R^p, also known as Euclidean space, is a mathematical concept that describes a p-dimensional space, where p is any positive integer. In simpler terms, R^p is a space with p coordinates, or dimensions, where points can be located.

Why is it important for every open subset of R^p to be the union of countable collections?

This property, known as the Lindelöf property, is important because it ensures that every open subset of R^p can be covered by a countable number of smaller open subsets. This is useful for many mathematical applications, such as proving the convergence of sequences and series.

What is a countable collection?

A countable collection is a set of elements that can be put into a one-to-one correspondence with the natural numbers (1, 2, 3, ...). In other words, the elements in a countable collection can be counted and listed in a specific order.

Can this property be extended to other topological spaces?

Yes, this property can be extended to any topological space that has the Lindelöf property. This includes metric spaces, normed spaces, and other topological spaces. However, there are also topological spaces that do not have this property, such as the Sorgenfrey plane.

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