# d(f(g(p )), f(g(q))) = d(g(p ), g(q)) = d(p, Q).

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1 10.2. (continued) As we did in Example 5, we may compose any two isometries f and g to obtain new mappings f g: E E, (f g)(p ) = f(g(p )), g f: E E, (g f)(p ) = g(f(p )). These are both isometries, the first because d(f(g(p )), f(g(q))) = d(g(p ), g(q)) = d(p, Q). Moreover, the parity of f g is obtained in the obvious way: The five examples provide us with the table even odd even even odd odd odd even isometry: identity rotation reflection translation glide reflection parity: even even odd even odd fixed points: E O l Classification by fixed points Main theorem. Any isometry of the plane is one of the ones in the table, namely: the identity, a rotation, a translation, a reflection, or a glide reflection. Let us first record two consequences of this theorem. Corollary 1. The composition of any two rotations, or of any two reflections, must be a rotation, a translation or the identity. Corollary 2. Any isometry f: E E is a bijective mapping and therefore has an inverse f 1 : E E. This inverse can be described explicitly case by case, and is always another isometry. For example, (Rot O,θ ) 1 = Rot O, θ (or Rot O,2π θ if one prefers to use angles between 0 and 2π), whereas (Ref l ) 1 = Ref l is self-inverse. We shall prove the main theorem by breaking it down into a number of simpler steps. Propositon 1. If A, B are distinct fixed points of an isometry f then f is the identity or a reflection in the line l = AB. Proof. We shall first prove that f fixes every point of l. Let P l. Construct circles, one with centre A, and one with centre B, passing through P. The two circles must touch at P, meaning that they intersect in a single point. If not, there are two points of intersection P, Q and the perpendicular bisector of the chord P Q must pass though both centres, contradicting the fact that P l. (Recall that there is a unique circle through 3 non-collinear points, proving the well-known fact that two circles cannot meet in more than 2 points!) We now know that 40

2 d(f(p ), A) = d(f(p ), f(a)) = d(p, A), and similarly with B in place of A. Therefore f(p ) lies on both circles, and f(p ) = P. Suppose that there exists a point not on l such that f(p ) = P. Drop a perpendicular from P to l with foot D l. Then f fixes every point on m = P D. But now every point Q E can be connected by a line that meets l and m in different points (any line through Q will do except for three). By the argument above, Q is also fixed. Therefore f is the identity. Finally, suppose that f is not the identity, and take P to be a point not on l. Construct circles though P with centres A and B and apply the same trick as before. This time, our assumption forces f(p ) = Q, where P Q is a chord whose perpendicular bisector is l. It follows that f(p ) is reflection of P in l, and f = Ref l. Corollary 3. An isometry is uniquely determined by its action on three non-collinear points. Proof. Suppose that two isometries f, g both move P, Q, R to P, Q, R. Then h = f 1 g fixes these points. Assuming R does not lie on the same line as P and Q, it follows that h = id and so f = g. Proposition 2. Let O E. Any isometry f such that f(o) = O and for which O is the only fixed point is a rotation: f = Rot O,θ for some angle θ (not a multiple of 2π). Sketch proof. A point P distinct from O maps to f(p ) = P. Both P and P lie on a circle C with centre O. A third point Q = Rot O,α (P ) on C maps to f(q) = Q, which is either Rot O,α (P ) or Rot O, α (P ); suppose the latter: P' M' M Q O Let M be the point of C such that OM bisects QOQ, and M = f(m) its image. By considering P QM, congruent to P Q M, one proves that M = M is a fixed point, contrary to hypothesis. 41

3 Proposition 3. Any isometry without fixed points is a glide reflection or a translation. Proof. Given such an f, fix A E and let M be the midpoint of AA where A = f(a). Let g = Rot M,π so that A is a fixed point of h = g f. By the previous propositions, h is the identity, a reflection or a rotation. Case 0. It cannot be the identity for then f = g 1 = g has a fixed point. Case 1. h = g f is a reflection in a line l. The latter cannot contain M for otherwise f = g h would fix M. Drop a perpendicular m from M to l, and consider the effect of f = g h on M. In fact, f is now acting on the yellow triangle as a glide reflection G. By Corollary 3, f must equal G : A M f M m A' f A Case 2. h is a rotation. It follows that f = g h is an even isometry preserving the orientation of every triangle. We now go back to the drawing board: replace g by g = Ref m where m is the perpendicular bisector of AA. Then h = g f again fixes A, is an odd isometry and must be a reflection. It follows that f = g h is the composition of two reflections. We shall see that such a composition is rotation about the point of intersection of the mirrors, unless they are parallel in which case (by q. 5 of Sheet 7) it is a translation. 11 Groups of Isometries The concept of a group Let G denote the set of isometries f: E E of the Euclidean plane E. We have defined a way of multiplying two isometries f, g, regarded as abstract symbols, to form their composition f g. This operation satisfies the so-called associative law (f g) h = f (g h), which asserts that brackets are not essential. The equation is true because both sides applied to P yield the same point f(g(h(p ))). 42

4 There is also an identity element for the multiplication, namely the lazy isometry id that satisfies id f = f = f id, f G. Moreover, given any isometry f G we know that there exists an inverse f 1 G characterized by the equations f f 1 = id = f 1 f. They key point here is that f 1 exists not just as a mapping E E of sets, but it is also an isometry. If there were any doubt of this last fact, we merely observe that d(f 1 (P ), f 1 (Q)) = d(f(f 1 (P )), f(f 1 (Q))) = d(p, Q), the first equality because f itself preserves distance. A set G with the above structure is called a group, and this is a key concept that occurs in many different branches of mathematics and other subjects, including the physics of elementary particles and developmental psychology. The set G of isometries (we can now say the group of isometries ) obviously has infinitely many elements. A simple example of a finite group consists of the set F = {1, i, 1, i} C ( F because it has 4 elements), with the usual multiplication of complex numbers. All the group rules are satsified. For example, the inverse of i is i 1 = 1/i = i, and 1 is self-inverse. ( ) a b Another infinite example is the set of 2 2 matrices A =, with non-zero c d determinant = ad bc. We can multiply them since det(ab) = (det A)(det B) remains non-zero. Matrix multiplication obeys ( the associative ) law (though this is not 1 0 obvious), and there is an identity matrix I =. The inverse of A is given by 0 1 ( ) ( ) 1 a b d/ b/ =. c d c/ d/ Denoting this matrix by A 1, it is easy to verify that both AA 1 and A 1 A equal I. We shall show that this example is relevant to the study of isometries, at least those that fix a given point. Let us return to isometries, and choose a point O in the plane. Let f be an isometry, and set A = f(o), and let h = Tran OA. Then h 1 = Tran AO and g = h 1 f fixes O. It follows that any element of G can be expressed as h g, where g belongs to the set of isometries that fix O. G O = {f G : f(o) = O} 43

5 The set G O is a group just like G is: just observe that if g and g fix O so does g g, and g 1 (O) = g 1 (g(o)) = O. Indeed, G O is an example of a subgroup of G, being a subset closed under the group operations. Another subgroup is the set H of all translations, and G is in some sense formed by combining H and G O. In practice, we already know what the elements of G O are: the identity, all rotations centred at O and all reflections whose mirror contains O. We can also assert that the identity and the set of rotations forms a subgroup. However, at this juncture we wish to describe elements of G O more analytically Linear isometries Choose perpendicular axes centred at O, so that we can represent any point P of E by its Cartesian coordinates (x, y), or equivalently by the vector ( ) v = OP x =. y Note that d(o, P ) 2 = OP 2 = x 2 + y 2 = v 2 = v v. In the same way, we denote by f(v) the column vector version of f(p ). Theorem. Any element of G O is a linear mapping, that is f(v + w) = f(v) + f(w) and f(λv) = λf(v) for λ R. Proof. The isometry condition is d(f(p ), f(q)) = d(p, Q). In vector language this translates into the equation f(v) f(w) = v w, which when squared and expanded becomes But we also know that f(v) 2 2f(v) f(w) + f(w) 2 = v 2 2v w + w 2. f(v) 2 = d(o, f(p )) 2 = OP 2 = v 2, and similarly for w in place of v. Therefore, Now consider the unit vectors v 1 = i = f(v) f(w) = v w ( ) 1, v 0 2 = j = ( ) 0 1 parallel to the axes. These two form an orthonormal basis, meaning that { 1, i = j v i v j = 0, i j, 44

6 and an arbitrary vector equals v = xv 1 + yv 2, with x = v v 1, y = v v 2. It follows from the boxed formula that f(v 1 ) and f(v 2 ) also form an orthonormal basis, and so f(v) = Xf(v 1 ) + Y f(v 2 ), where X = f(v) f(v 1 ) = v v 1 = x, and Y = y. Thus, f(xv 1 + yv 2 ) = xf(v 1 ) + yf(v 2 ), and the fact that f is linear follows immediately (well, almost!). If we set f(v 1 ) = ( ) a, f(v c 2 ) = then the last equation of the proof shows that f maps ( ) ( ) ( ) ( x a b a b x + y = y c d c d ( ) b, d ) ( ) x = A y ( ) x. y This means that to apply the isometry f, we need to multiply the column vector v by the matrix A defined by the last equality. But we also know that the columns of A are orthonormal: they satisfy This can be expressed by the equation ( ) ( ) a c a b = b d c d a 2 + c 2 = 1 = b 2 + d 2 and ab + cd = 0. ( ) 1 0, or A T A = I. 0 1 Definition. A square matrix A that satisfies A T A = I is called orthogonal. It follows from the theory of determinants that if A is an orthogonal matrix then det A = ±1. This in turn tells us that the inverse matrix A 1 exists and equals A T, so it is also true that AA T = I. 45

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