The symmetry operation corresponding to a rotation followed by an inversion operation, also called a rotoinversion. This operation is denoted for an improper rotation by , so the crystallography restriction gives only , , , , for crystals. The mirror plane symmetry operation is , etc., which is equivalent to .

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Weisstein, Eric W. "Improper Rotation." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/ImproperRotation.html