Dedekind Cuts & the Real Line: A Countable Set?

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In summary, Dedekind cuts can consist of both rational and irrational numbers. While some cuts may only produce a countable set, others can produce the entire real line. This is demonstrated by the fact that there are Dedekind cuts that are not at rational numbers, such as the example given by taking a real number A and defining a cut using its decimal expansion. Therefore, it is important to consider all possibilities when thinking about Dedekind cuts.
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cragar
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If every Dedekind cut is at a rational it seems that these cuts would only produce a countable set and would not produce the whole real line. So how should I think about it.
 
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cragar said:
If every Dedekind cut is at a rational it seems that these cuts would only produce a countable set and would not produce the whole real line. So how should I think about it.

The point is that there are Dedekind cuts that are NOT at rationals.
For example, take any real number A and consider its decimal expansion. A sequence of rational numbers An can be defined by taking n terms of the expansion. Let the cut be defined by all rationals less than any term in the sequence. This cut gives the real number A.
 
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Even though all Dedekind cuts consist of only rational numbers, all are not rational cuts.
T = { x [itex]\in[/itex] Q: x^2 < 2 or x < 0 } is a dedekind cut, you can check that all the properties hold, but it can not be a rational cut.
 

Related to Dedekind Cuts & the Real Line: A Countable Set?

1. What is a Dedekind Cut?

A Dedekind Cut is a mathematical concept introduced by German mathematician Richard Dedekind to define the real numbers. It involves partitioning the rational numbers into two non-empty sets, with one set containing all the rational numbers less than a certain real number, and the other set containing all the rational numbers greater than or equal to that real number.

2. How does a Dedekind Cut relate to the real line?

A Dedekind Cut is used to define the real numbers, which can be represented as points on a continuous line known as the real line. Each Dedekind Cut represents a unique real number on the real line.

3. What is a countable set?

A countable set is a set that can be put into a one-to-one correspondence with the set of natural numbers. This means that the elements of the set can be counted and listed in a specific order, with each element having a unique position in the list.

4. How does a countable set relate to Dedekind Cuts and the real line?

A countable set can be used to represent the rational numbers, which are the basis for Dedekind Cuts. The real line, which is defined by Dedekind Cuts, includes both rational and irrational numbers. Therefore, a countable set is a fundamental component in understanding the real line and the concept of Dedekind Cuts.

5. Why is understanding Dedekind Cuts and the real line important?

Understanding Dedekind Cuts and the real line is important because it allows us to define and understand the real numbers, which are essential in many areas of mathematics and science. The concept of Dedekind Cuts also has applications in fields such as computer science and economics.

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