What are the symmetries of an equilateral triangle?

Transcription

1 What are the symmetries of an equilateral triangle? A In order to answer this question precisely, we need to agree on what the word symmetry means.

2 What are the symmetries of an equilateral triangle? A For our purposes, a symmetry of the triangle will be a rigid motion of the plane (i.e., a motion which preserves distances) which also maps the triangle to itself. Note, a symmetry can interchange some of the sides and vertices.

3 So, what are some symmetries? How can we describe them? What is good notation for them? A

4 Rotate counterclockise, 120 about the center. A

5 Note this is the following map (function): A A We can think of this as a function on the vertices: A,, A. ( ) A We might denote this by: A We also may denote this map by R 120.

6 Rotate counterclockise, 240 about the center. This is the map (function): A A We can think of this as a function on the vertices: A, A,. ( ) A We might denote this by: A We also may denote this map by R 240.

7 Reflect about the perpendicular bisector of A : A

8 Reflect about the perpendicular bisector of A, This is the map (function): A A We can think of this as a function on the vertices: A, A,. ( ) A We might denote this by: A We also may denote this map by F to indicate the reflection is the one fixing.

9 Reflect about the perpendicular bisector of, This is the map (function): A A We can think of this as a function on the vertices: A A,,. ( ) A We might denote this by: A We also may denote this map by F A to indicate the reflection is the one fixing A.

10 Reflect about the perpendicular bisector of A, This is the map (function): A A We can think of this as a function on the vertices: A,, A. ( ) A We might denote this by: A We also may denote this map by F to indicate the reflection is the one fixing.

11 The identity map of the plane: (takes every point to itself). This is the map (function): A A We can think of this as a function on the vertices: A A,,. ( ) A We might denote this by: A We also may denote this map by Id or 1. Note, we might also denote this as R 0, since it is a rotation through 0. However it is NT a reflection. (WHY NT??!!)

12 So far we have 6 symmetries 3 rotations, R 0, R 120, R 240, and 3 reflections, F A, F, F. A Are there any more?? Why or why not??

13 In fact these are all the symmetries of the triangle. We can see this from our ( notation in) which we write each of A these maps in the form. Note there are three X Y Z choices for X (i.e., X can be any of A,,,). Having made a choice for X there are two choices for Y. Then Z is the remaining vertex. Thus there are at most = 6 possible symmetries. Since we have seen each possible rearrangement of A,. is indeed a symmetry, we see these are all the symmetries.

14 Notice these symmetries are maps, i.e., functions, from the plane to itself, i.e., each has the form f : R 2 R 2. Thus we can compose symmetries as functions: If f 1, f 2 are symmetries then f 2 f 1 (x) = f 2 (f 1 (x)), is also a rigid motion. Notice, the composition must also be a symmetry of the triangle. For example, R 120 F =?? It must be one of our 6 symmetries. an we tell, without computing whether it is a rotation or reflection?? Why?? What about the composition of two reflections?

15 R 120 F, we can view this composition as follows: F A A A R 120 So, R 120 F = F. A

16 We use our other ( notation: ) A R 120 F = A ( ) A = A ( ) A = F A

17 Is R 120 F = F = R 120? Let s look: F R 120 : R 120 A A F A A So F R 120 = F A F = R 120 F.

18 So on our set of symmetries S = {R 0, R 120, R 240, F A, F, F }, we get a way of combining any two to create a third, i.e., we get an operation on S. (Just like addition is an operation on the integers.) We will call this operation multiplication on S. We can make a multiplication table, or ayley Table. So far we have: R 0 R 120 R 240 F A F F R 0 R 0 R 120 R 240 F A F F R 120 R 120 F R 240 R 240 F A F F A F F F F A Notice we have already seen F R 120 R 120 F, so this operation is non-commutative.

19 Now we fill in the rest: (check) R 0 R 120 R 240 F A F F R 0 R 0 R 120 R 240 F A F F R 120 R 120 R 240 R 0 F F A F R 240 R 240 R 0 R 120 F F F A F A F A F F R 0 R 120 R 240 F F F F A R 240 R 0 R 120 F F F A F R 120 R 240 R 0 We make note of several things about this table: (i) Every symmetry appears exactly once in each row and in each column; (ii) Every symmetry has an opposite or inverse symmetry; (iii) Less clear from the table: If f, g, h are symmetries of our triangle (f g) h = f (g h). UT THIS IS A FAT AUT FUNTINS (and we already know it!!).

20 We learn in High School Algebra, and again in alculus (and re-learned in h. 0) f (g(h(x))) = f (g h)(x) = (f g) h(x). We call this Associativity.

21 bservations

22 So our set S of symmetries has the following property: (i) There is a binary operation on S, i.e., a way to combine two members of S to get another one,(composition) we write ψϕ instead of ψ ϕ; (ii) This operation is associative: ψ(σϕ) = (ψσ)ϕ. (iii) There is an identity element for the operation, i.e., an element σ so that σψ = ψσ = ψ, for all ψ; (The identity is R 0.) (iv) Every element has an inverse Given ψ S there is a σ S so that ψσ = σψ = R 0.

23 There are other examples of sets, say, G satisfying (i)-(iv) The integers Z, with the operation + (i), is associative (ii), the integer 0 is the additive identity (iii) and for any n we have n + ( n) = 0. (iv). The positive real numbers R with multiplication GL(2, R) the set of all 2 2 invertible real matrices with the operation of matrix multiplication. A set G with a closed binary operation,, satisfying (i)-(iv) is called a group.