A question about dedekind cuts

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SUMMARY

The discussion centers on the mathematical concept of Dedekind cuts, specifically addressing the product of the cuts 1* and -1*. The cut 1* represents all rational numbers less than 1, while -1* includes all rational numbers less than -1. The product of these two cuts results in the set of all rational numbers, demonstrating that every positive rational number can be expressed as the product of elements from these cuts. This exercise illustrates the limitations of defining the product of two Dedekind cuts in this manner.

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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

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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.)
 

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