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A rotation in the plane can be formed by composing a pair of reflections. First reflect a point P to its image P′ on the other side of line L 1. Then reflect P′ to its image P′′ on the other side of line L 2. If lines L 1 and L 2 make an angle θ with one another, then points P and P′′ will make an angle 2θ around point O, the ...
If any of the angles are the same then the planes are not unique, as in four dimensions with an isoclinic rotation. In even dimensions (n = 2, 4, 6...) there are up to n / 2 planes of rotation span the space, so a general rotation rotates all points except the origin which is the only fixed point. In odd dimensions (n = 3, 5, 7, ...
Thus we can build an n × n rotation matrix by starting with a 2 × 2 matrix, aiming its fixed axis on S 2 (the ordinary sphere in three-dimensional space), aiming the resulting rotation on S 3, and so on up through S n−1. A point on S n can be selected using n numbers, so we again have 1 / 2 n(n − 1) numbers to describe any n × n ...
The rotation group is a point stabilizer in a broader group of (orientation-preserving) motions. For a particular rotation: The axis of rotation is a line of its fixed points. They exist only in n = 3. The plane of rotation is a plane that is invariant under the rotation. Unlike the axis, its points are not fixed themselves.
Elements of 3D Plane-based GA, which includes planes, lines, and points. All elements are constructed from reflections in planes. Lines are a special case of rotations. Plane-based geometric algebra is an application of Clifford algebra to modelling planes, lines, points, and rigid transformations.
If the rotation angles are unequal (α ≠ β), R is sometimes termed a "double rotation". In that case of a double rotation, A and B are the only pair of invariant planes, and half-lines from the origin in A, B are displaced through α and β respectively, and half-lines from the origin not in A or B are displaced through angles strictly ...
There are thus 10 two-dimensional crystallographic point groups: C 1, C 2, C 3, C 4, C 6, D 1, D 2, D 3, D 4, D 6; The groups may be constructed as follows: C n. Generated by an element also called C n, which corresponds to a rotation by angle 2π/n.
If k = m, then such a transformation is known as a point reflection, or an inversion through a point. On the plane (m = 2), a point reflection is the same as a half-turn (180°) rotation; see below. Antipodal symmetry is an alternative name for a point reflection symmetry through the origin. [14]