Communications in Applied Geometry. Volume 1, Number 1 (2011) pp. 7-16 © Research India Publications https://dx.doi.org/10.37622/CAG/1.1.2011.7-16 Projections and Dimensions Euich Miztani JEIN Institute for Fundamental Science (JIFS) VBL Kyoto University, Yoshida-honmachi, sakyo-ku, Kyoto, 600-8813, Japan E-mail: [email protected] or [email protected] Abstract In this paper, we consider a relationship between projection and dimension in geometry. We discuss the relation from projective geometrical viewpoint by paying attention to higher and lower dimensional spaces. Then, we categorize those projections among different dimensional ones by their characteristics. We also express the classification by group theory and verify that there exists the automorphism. Keywords: freedom degree, higher dimensional space, lower dimensional space, projective geometry, homogeneous coordinates 2000 Mathematics Subject Classification: 51N15, 51N25, 51N35. Introduction This paper is mainly focused on a viewpoint of geometry projected into higher and lower dimensional spaces. First of all, about projection into higher dimensional space, for example, mapping any point in the space of the first dimension into second dimension, we will commonly consider that it is kept on point. Similarly, the point is point all in dimensional spaces. However, we could unnecessarily say any point mapped into higher dimensional space is point. As such a case, for example, we could pick up that x a indicates a point in one-dimensional space but x a in two- dimensional space is considered equation of a straight line. We discuss it from the section 2.1 to 2.3 in detail. Contrarily, thinking of projection into lower dimensional space, we discuss it with homogeneous coordinates. For example, let us think of an equation 2 2 x x x y x x f ( x , y ) x y 1 0 . If substituting 1 / 0 and 2 / 0 for the equation, then 8 Euich Miztani 2 2 2 2 2 it is expanded as x1 / x 0 x 2 / x 0 1 0 . Hence x1 x2 x0 0 . The expanded 2 2 2 equation x1 x2 x0 0 is a quadric surface in the three-dimensional space though the original equation f ( x , y ) x 2 y 2 1 0 indicates a circle in the two-dimensional space. Such a mapping between homogeneous and non-homogeneous coordinates could be considered the projection into lower dimensional space from our viewpoint. We further discuss it in the section 2.4. Finally, we discuss that a series of projections among higher and lower dimensional spaces makes a group. Especially, conclusion in the section 2.5 will request them necessity of description by transformation group. In Chapter 3, we discuss how to describe what is discussed in Chapter 2. Concept and Definition Degree of Freedom First of all, we designate any point in the orthogonal coordinates. Assuming the point x a is mapped into the two-dimension, it will be commonly considered as point also in the two-dimension as shown in Figure 1. However, let us contemplate the mapped point. It originally has only one-degree of freedom along the x-axis. Nonetheless, the second degree of freedom has already been assumed in the above two-dimension; Because the location of the point in the two-dimension is (x, y) a,0 as shown in Figure 1, the second degree of freedom is already given as 0 or the y-coordinate is set as 0. Figure 1 If the mapped point is related to the second dimension by keeping its original one- degree of freedom, we cannot decide the y-coordinate. To avoid the decision, the mapped point should occupy overall y-coordinates to keep its original 1-dimension’s character as shown in Figure 2. It is straight line which is parallel to the y-axis because it is only the way to exclude the second degree of freedom. Therefore, x a is point in the 1-dimension, but equation of a straight line in the two-dimension. It is uncontradicted in orthodox mathematics. Projections and Dimensions 9 Figure 2 Furthermore, about mapping we have discussed, in other words, we could say like that any point of the one-dimension is projected into the two-dimension in the broadest sense of the word(; the term projection is generally used for mapping from higher to lower dimensional space or within the same dimension though.) Henceforth, we say project instead of map in a series of the process. The Projection of Any Point in the Coordinate Let us think about and define the above understanding with easy examples of any point in the one and two-dimension projected into the two and three-dimension in accordance with the style of Euclid’s element. About any point in the one-dimensional coordinate; Definition 1. If projecting any point of the one-dimension into the two-dimension, then it is a line that is parallel to y-axis of the two-dimension. For example, it is the equation of a straight line x a as discussed in the section 2.1. Definition 2. If projecting any point of the one-dimension into the three-dimension, then it is a plane that is parallel to y-z plane in the three-dimension. For example, it is x a where a is the distance between the plane and y-z plane as shown in Figure 3. Figure 3 10 Euich Miztani About any point in the two-dimensional space; Definition 3. If projecting any point of the two-dimension into the three-dimension, then it is a line that is parallel to z-axis of the 3-dimension. For example, it is x a , y b where the coordinates of the line is (x, y) (a,b) as shown in Figure 4. Figure 4 Definition 4. If projecting any line(; any set of points) of the two-dimension into the three-dimension, then it is a plane in the three-dimension. For example, it is y x as shown in Figure 5. Figure 5 Application for All Other Coordinates It is possible that all other kinds of coordinates are defined by the same way. For example, projection of any point of two-dimensional polar coordinates into three- dimensional spherical coordinates is expressed by a circle with attention to no degree of freedom of the variable as shown in Figure 6. Projections and Dimensions 11 Figure 6 Hence, in the polar and sphere coordinates, we define any point of one and two- dimensional coordinate according to our discussion in the section 2.2; Definition 5. If projecting any point of one-dimensional coordinate into two- dimensional polar ones, then it is a circle. Definition 6. If projecting any point of one-dimensional coordinate into three- dimensional spherical ones, then it is a sphere. About any point of the two-dimensional polar coordinates; Definition 7. If projecting any point of two-dimensional polar coordinates into three- dimensional spherical ones, then it is a circle as shown in Figure 6. Definition 8. If projecting any circle of two-dimensional polar coordinates into three- dimensional spherical ones, then it is a sphere. Projection into Lower Dimensions Contrarily, let us think of projection into lower dimensional space such from our viewpoint. As for already known case, projection of a curve of the three-dimensional orthogonal coordinates into the x-y plane as in shown Figure 7 seems to be such a case. It will be certainly a common image of projective geometry. However, from our viewpoint, it is not projected into lower dimensional space. Because, the mapping is f (x, y, z) g(x, y,0) ; it is projected within the same dimensional space. 12 Euich Miztani Figure 7 Instead, we propose mapping between homogeneous coordinates and non- homogeneous ones as an example. To explain what it suggests, let us think of a 2 2 quadric equation f ( x, y) x y 1 0 . Substituting x x0 / x2 and y x1 / x2 for the 2 2 2 2 2 equation, then x0 / x2 x1 / x2 1 0 , x 0 x1 x 2 0 ( g ( x 0 , x1 , x 2 )) . The expanded equation g ( x 0 , x1 , x 2 ) indicates a quadric surface in three-dimensional space, but the original equation f ( x , y ) does a circle in two-dimensional space. The original equation merely shows a part of its character though, it actually has a quite different character in higher dimensional space. Now, let us take the projective transformation from another viewpoint. Assume that g ( x 0 , x1 , x 2 ) is projected into two-dimensional space, 2 2 then it is x / x x / x 1 x 2 y 2 1 f ( x, y ) . It means that f ( x , y ) virtually has 1 0 2 0 three variables or three-degrees of freedom in the two-dimension; g ( x , x , x ) keeps its 0 1 2 original three-degrees of freedom also after projecting into the two-dimension. Therefore, the relation between g ( x , x , x ) and f ( x , y ) is projection into lower 0 1 2 dimensional space from our viewpoint. Thus, we take mapping from homogeneous coordinates to non-homogeneous ones as projection into lower dimensions. Classification Thinking of these projections among higher and lower dimensional spaces, we have an opportunity to classify such projections by the corresponding manner of Erlangen program. It is as follows; 1. Projection into higher dimensional space; Our discussion on the section 2.1 to 2.3 comes under this category. 2. Projection into lower dimensional space; Mapping from homogeneous coordinates to non-homogeneous ones as discusses on the section 2.4.