ABSTRACT
Rotations are the examples of continuous transformations, since a rotation by any finite angle can be viewed as an infinite sequence of infinitesimal rotations. There exist also discrete transformations, which cannot be presented as a series of infinitesimal steps. The space inversion r → −r is an example of such a transformation. The corresponding group is finite and contains only two elements: the unit element I and an element P , satisfying a condition P 2 = 1 (to use the space inversion transformation twice is equivalent to the identical transformation).
