Lecture 5 35 x = Ix y z Iy Iz Rotation—one fixed point x y z Ix Iy Iz Translation—no fixed points Figure 1.21. Orientation preserving isometries. orientation is reversed. So for two pairs of distinct points a, b and a , b such that the distances between a and b and between a and b coincide, there are exactly two isometries which map a to a and b to b one of these will be orientation preserving, the other orientation reversing. Passing to algebraic descriptions, notice that any isometry I must carry lines to lines, since as we saw last time, three points in the plane are collinear iff the triangle inequality becomes degenerate. Thus it is an aﬃne map—that is, a composition of a linear map and a translation—so it may be written as I : x → Ax + b, where b ∈ R2 and A is a 2 × 2 matrix. In fact, A must be orthogonal, which means that we can write things in terms of the complex plane C and get (in the orientation preserving case) I : z → az + b, where a, b ∈ C and |a| = 1. In the orientation reversing case, we have I : z → a¯ + b. Using the preceding discussion, we can now classify any isometry of the Euclidean plane as belonging to one of four types, depending on whether it preserves or reverses orientation, and whether or not it has a fixed point. Case 1 : An orientation preserving isometry which possesses a fixed point is a rotation. Let x be the fixed point, Ix = x. Fix another point y both y and Iy lie on a circle of radius d(x, y) around x. The rotation about x which takes y to Iy satisfies these criteria, which are enough to uniquely determine I given that it preserves orientation hence I is exactly this rotation.

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