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Homework Statement
Problem:
Dedekind defined a cut, section or partition in the rational number system as a separation of all rational numbers into two classes or sets called L (the left-hand class) and R (the right-hand class) having the following properties.:
I. The classes are non-empty (i.e. at least one number belonds to each class).
II. Every rational number is in one class or the other.
III. Every number in L is less than every number in R.
Prove each of the following statements.:
(a) There cannot be a largest number in L and a smallest number in R.
(b) It is possible for L to have a largest number and for R to have no smallest number. What type of number does the cut define in this case?
(c) It is possible for L to have no largest number and for R to have a smallest number. What type of number does the cut define in this case?
(d) It is possible for L to have no largest number and for R to have no smallest number. What type of number does the cut define in this case?
(Textbook's) Solution:
(a) Let a be the largest rational number in L and b the smallest rational number in R. Then either a = b or a < b. We cannot have a = b, since, by definition of the cut, every number in L is less than every number in R. We cannot have a < b, since by [another problem in the textbook], ½ (a + b) is a rational number which would be greater than a (and so would have to be in R) but less than b (and so would have to be in L), and, by definition, a rational number cannot belong to both L and R.
(b) As an indication of the possibility, let L contain the number 2/3 and all rational numbers less than 2/3, while R contains all rational numbers greater than 2/3. In this case, the cut defines the rational number 2/3. A similar argument replacing 2/3 by any other rational number shows that in such a case the cut defines a rational number.
(c) As an indication of the possibility, let L contain all rational numbers less than 2/3, while R contains all rational numbers greater than 2/3. This cut also defines the rational number 2/3. A similar argument shows that this cut always defines a rational number.
(d) As an indication of the possibility, let L consist of all negative rational numbers and all positive rational numbers whose squares are less than 2, while R consists of all positive numbers whose squares are greater than 2. We can show that if a is any number of the L class, there is always a larger number of the L class, while if b is any number of the R class, there is always a smaller number of the R class. A cut of this type defines an irrational number.
From parts (b), (c) and (d), it follows that every cut in the rational number system, called a Dedekind cut, defines either a rational or an irrational number.
By use of Dedekind cuts, we can define operations (addition, multiplication, etc.) with irrational numbers.
Just in case I made a mistake when typing the problem and its solution, I am attaching the TheProblemAndSolution.jpg file.
Homework Equations
Definition of Dedekind cut.
The Attempt at a Solution
Based on what the question asked, I just wanted to know if, for part (c), it should have said that R contains all rational numbers greater than or equal to 2/3 instead of just greater than 3/2?
Or is there something I'm not grasping?