Delian Problem and Other Construction Problems
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Delian problem and other construction problems by LIU Rui Ling (19220642) A thesis submitted in partial fulfillment of the requirements for the degree of Bachelor of Science (Honours) in Mathematics and Statistics at Hong Kong Baptist University 14 January 2021 Acknowledgements Thanks to my supervisor, Dr. Sun Pak Kiu provided some useful reading materials for me so that I have a clear idea of how completing this thesis. Also, I am much obliged to Dr. Sun for his friendly reminder and support. ______________________ Signature of Student LIU Rui Ling Student Name Department of Mathematics Hong Kong Baptist University Date: 14 January 2021 1 Delian problem and other construction problems LIU Rui Ling (19220642) Department of Mathematics Abstract In this thesis, I will discuss the problem of duplication of a cube, which is one of the three geometrical construction problems in ancient Greek mathematics. The problem of doubling the cube refers to whether we can use a straightedge with no scale and compass to construct a cube so that its volume is equal to twice the original one. In chapter one, I will introduce the history of the problem of doubling the cube. In chapter two, I will discuss the straightedge and compass construction. Therefore, I will explain that how to construct numbers by using straightedge and compass construction. In chapter three, I will discuss the fields. In chapter four, I will discuss the field extensions, including the vector space, the field extensions and the algebraic extension. In chapter five, I will prove that why we cannot solve the problem of doubling the cube by using the straightedge and compass construction. 2 Table of Contents Acknowledgements ........................................................................................................ 1 Abstract .......................................................................................................................... 2 Chapter 1The history of the problem of doubling the cube ........................................... 4 Chapter 2 Constructible Numbers .................................................................................. 5 2.1 The straightedge and compass construction .......................................................... 5 Chapter 3 Fields ........................................................................................................... 10 Chapter 4 The field extensions .................................................................................... 11 4.1 The vector space .................................................................................................. 11 4.2 The field extensions ............................................................................................. 13 4.3 The algebraic extension ....................................................................................... 15 Chapter 5 Doubling the cube ....................................................................................... 17 Chapter 6 Conclusion ................................................................................................... 19 References .................................................................................................................... 20 3 Chapter 1 The history of the problem of doubling the cube Delian problem, also called the problem of doubling the cube, is one of the three geometric construction problems raised by the ancient Greeks 2,400 years ago. The problem of doubling the cube is to construct a cube which its volume is equal to twice the original one. The reason why this question is difficult to solve is that there are restrictions on the drawing tools. The ancient Greeks emphasized that only straightedge with no scales and compasses could be used for geometric drawing. There is a myth about the origin of the problem of doubling the cube. [1] The myth is that a plague broke out in the island of Delos. A quarter of the population of Delos died of the plague. The islanders went to the temple to ask for Apollo's will. The oracle replied that if they want to stop the plague, they should double the volume of altar in the Apollo’s temple. The islanders doubled the length of each side. The new altar was finished, and the plague continued to spread. They felt confused. They asked the famous philosopher and mathematician Plato about the reason why the oracle was not working. Plato pointed out the problem. They doubled each side of the altar, so that the volume would become eight times the original size instead of doubling. At this time, everyone suddenly realized. So, if the volume is to be doubled, how many should the length of each side increase? This is the problem of doubling the cube among the three famous construction problems in ancient Greece. Since then the problem of doubling the cube also called the Delian problem. Since the appearance of the problem of the doubling the cube, it has attracted many mathematicians to study. In 1837, a French mathematician, Pierre Wantzel proved that we could not use the straightedge and compass construction to solve the Delian problem. 4 Chapter 2 Constructible Numbers 2.1 The straightedge and compass construction Straightedge and compass construction is a mathematical problem that originated in ancient Greece. Only compasses and straightedges can be used to solve a given problem in a limited number of steps. In the straightedge and compass construction, the definition of straightedge and compass as below: A straightedge is unmarked, it can only be used for drawing a line segment through any two constructed points. A compass is used for drawing a circle with a constructed point O as the center and a constructed line segment r (r>0) as the radius. For two arbitrary points, point A and point B, we define |AB | as their distance. First, finding two points A and B. The line determined by these two points is the x- axis. Second, drawing a circle with A as the center and |AB | as the radius by using a compass. Final, drawing a circle with B as the center and |AB | as the radius by using a compass. Two circles which we had drawn intersect at point C and point D, and the straight line determined by these two points is the y-axis. The two line segments, that are AB and CD intersect at a point O, which is the origin of the coordinate system. We define the length of |OA| as 1 unit, and the coordinate of O is (0,0). So, we can draw a line segment is 1 unit of length at the beginning. See Figure 2.1. Definition 2.1.1 The constructible number: The constructible number refers the real number that can be drawn in a limit number of steps by using a straightedge. In the case of a given |α| (α∈ ℝ) unit length, if the line segment could be drawn by using a straightedge, then the number α is a constructible. [2] 5 All the constructing steps can be reduced to the following five basic steps: Definition 2.1.2 The constructible point: The constructible point refers to as below: (a) An intersection point of 2 non-parallel constructible segments, or (b) An intersection point of 1 constructible segment and a constructible circle, or (c) An intersection point of 2 constructible circles. Definition 2.1.3 The constructible segment: The constructible segment refers to a line segment which two endpoints are constructible points. Definition 2.1.4 The constructible circle: The constructible circle refers to a circle with a constructible point as the center and a constructible segment as the radius. The length of each line segment is equivalent to a real number. Here we assume that "given a positive real number" means "given a line segment, and the length is equivalent to a given positive real number. Therefore, given two positive real numbers, we can construct the sum of the two numbers and difference. Given two positive real numbers and a line segment of length 1unit, then the product, quotient and square root of the two numbers could be constructed. So, for a given constructed line segment, we can obtain a constructible number by the five mathematical operation: addition, subtraction, multiplication, division and taking a square root. Let α, β>0, the five operation as show below: 1. α + β For two given line segments, line segment AB and line segment CD which the length of α and β respectively. Constructing a circle with point B as the center and line segment CD as the radius by a compass. Constructing a line segment AB, and the two intersection points of line segment AB and a constructed circle are point M and N respectively. See Figure 2.1.2. 6 2. α - β Two numbers that are α-β and α+β have a similar construction, but the line segment of CD is constructed in the opposite side. See Figure 2.1.3. 3. α x β For three given line segments, OP and OA which the length of α and βrespectively, and a line segment of length 1unit. Constructing a line segment OA with a length of β by using a compass. Let |OR|=1 unit, and point R lies on the line segment OA. Constructing a line l through point O and do not contain OA. Let |OP|=α, and a point P in the line l. Using the straightedge to construct a line segment RP and construct a line segment AQ from point A, such that a line segment PR and a line segment AQ is a pair of parallel lines, and the line segment AQ intersects the line l at point Q. Now, we have a pair of similar triangles, the triangle OPR is similar to the triangle OQA. According to the properties of similar triangle, we have 1 β = . 훼 |푂푄| So |OQ|=αβ. See Figure 2.1.4. 7 4. α / β Two numbers that are α/β and αβ have a similar construction. For three given line segments, OQ and OA which the length of α and β (whereβ≠0) respectively, and a line segment of length 1unit. Constructing a line segment OA with a length of β by using a compass. Let |OR|=1 unit, and point R lies on the line segment OA. Constructing a line l through point O and do not contain OA. Let |OQ|=α, and point Q lies on the line l. Using the straightedge to construct a line segment AQ and a line segment RP from a point R, such that the line segment AQ is parallel to the line segment RP, and the line segment RP intersects the line l at point P.