A question about dedekind cuts

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In summary, the product of 1* and -1* gives us all of Q because it includes all rational numbers, both positive and negative, and any number multiplied by a number in these cuts will also be included in the resulting set. This exercise demonstrates why this method cannot be used to define the product of two cuts.
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Artusartos
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Homework Statement



For number 6.4 in this link:

http://people.ischool.berkeley.edu/~johnsonb/Welcome_files/104/104hw2sum06.pdf

I don't understand why the product of 1* and -1* gives us all of Q...so can anybody please explain that to me?

Thanks in advance

Homework Equations





The Attempt at a Solution

 
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The cut 1* is the set of all rational numbers less than 1. The cut -1* is the set of all rational numbers less than -1. Their product is the set containing all products of numbers in those sets.

Let q be any positive rational number, let r be any negative rational number, less than -1 and let s= q/r, also a negative rational number. Then r is in -1*, s is in 1* and so q is in (1)*(-1)*. Thus all positive numbers are in this cut. Since if a is in a cut and if b< a, then b is also in the cut, it follows that all rational numbers are in it.

(I presume that the point of this exercise is to show why we cannot define the product of two cuts in this way.)
 

Related to A question about dedekind cuts

1. What are Dedekind cuts?

Dedekind cuts are a mathematical concept introduced by German mathematician Richard Dedekind. They are used to define the real numbers as a complete ordered field by dividing the set of rational numbers into two non-empty subsets.

2. How are Dedekind cuts used in mathematics?

Dedekind cuts are used to construct the real numbers, which are essential in various mathematical fields such as analysis, geometry, and physics. They are also used to prove many theorems and solve mathematical problems.

3. What is the significance of Dedekind cuts in mathematics?

Dedekind cuts provide a rigorous and formal way of defining the real numbers and bridging the gap between rational and irrational numbers. They also allow for the development of advanced mathematical concepts and theories.

4. How do Dedekind cuts relate to the concept of completeness?

Dedekind cuts are used to define the real numbers as a complete ordered field, meaning that every non-empty set of real numbers that is bounded above has a least upper bound. This concept of completeness is crucial in many mathematical proofs and constructions.

5. Can you give an example of a Dedekind cut?

One example of a Dedekind cut is the set of all rational numbers less than the square root of 2. This set does not contain the irrational number √2, but it is the cut-off point between rational and irrational numbers on the number line.

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