Reflection (mathematics)
In mathematics, reflection (also spelt reflexion) refers to an involutive automorphism of a space which leaves invariant a subspace of codimension 1. (This means that a two-dimensional (n dimensional) space is flipped around a one-dimensional (n-1 dimensional)axis within that space.)
Note that this applies to more than just Euclidean geometry. Reflections in affine geometry with respect to a given hyperplane is not unique, for example. Also, an inversion in inversive geometry is considered a reflection by this definition.
In algebra, especially relational algebra, a relation R is reflexive if, for any x,
- x R x
E.g. equality is reflexive because
- x = x.
See also: Coordinate rotations and reflections.
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