Inverse curve

In inversive geometry, an inverse curve of a given curve $C$ is the result of applying an inverse operation to $C$ . Specifically, with respect to a fixed circle with center $O$ and radius $k$ the inverse of a point $Q$ is the point $P$ for which $P$ lies on the ray $OQ$ and $OP \cdot OQ = k 2$ . The inverse of the curve $C$ is then the locus of $P$ as $Q$ runs over $C$ . The point $O$ in this construction is called the center of inversion, the circle the circle of inversion, and $k$ the radius of inversion.

An inversion applied twice is the identity transformation, so the inverse of an inverse curve with respect to the same circle is the original curve. Points on the circle of inversion are fixed by the inversion, so its inverse is itself.