Improper rotation

In geometry, an improper rotation,[1] also called rotation-reflection,[2] rotoreflection,[1] rotary reflection,[3] or rotoinversion[4] is, depending on context, a linear transformation or affine transformation which is the combination of a rotation about an axis and a reflection in a plane perpendicular to that axis.[5]

In 3D, equivalently it is the combination of a rotation and an inversion in a point on the axis.[1] Therefore it is also called a rotoinversion or rotary inversion. A three-dimensional symmetry that has only one fixed point is necessarily an improper rotation.[3]

In both cases the operations commute. Rotoreflection and rotoinversion are the same if they differ in angle of rotation by 180°, and the point of inversion is in the plane of reflection.

An improper rotation of an object thus produces a rotation of its mirror image. The axis is called the rotation-reflection axis.[6] This is called an n-fold improper rotation if the angle of rotation is 360°/n.[6] There are several different systems for naming individual improper rotations:

The direct subgroup of S2n, of index 2, is Cn, [n]+, or (nn), of order n, being the rotoreflection generator applied twice.

S2n for odd n contains an inversion, denoted Ci. But for even n S2n does not contain inversion. In general, if odd p is a divisor of n, then S2n/p is a subgroup of S2n. For example S4 is a subgroup of S12.

In a wider sense, an improper rotation may be defined as any indirect isometry; i.e., an element of E(3)\E+(3): thus it can also be a pure reflection in a plane, or have a glide plane. An indirect isometry is an affine transformation with an orthogonal matrix that has a determinant of −1.

A proper rotation is an ordinary rotation. In the wider sense, a proper rotation is defined as a direct isometry; i.e., an element of E+(3): it can also be the identity, a rotation with a translation along the axis, or a pure translation. A direct isometry is an affine transformation with an orthogonal matrix that has a determinant of 1.

In either the narrower or the wider senses, the composition of two improper rotations is a proper rotation, and the composition of an improper and a proper rotation is an improper rotation.

When studying the symmetry of a physical system under an improper rotation (e.g., if a system has a mirror symmetry plane), it is important to distinguish between vectors and pseudovectors (as well as scalars and pseudoscalars, and in general between tensors and pseudotensors), since the latter transform differently under proper and improper rotations (in 3 dimensions, pseudovectors are invariant under inversion).