A Porism Concerning Cyclic Quadrilaterals

Abstract EN

We present a geometric theorem on a porism about cyclic quadrilaterals, namely, the existence of an infinite number of cyclic quadrilaterals through four fixed collinear points once one exists.

Also, a technique of proving such properties with the use of pseudounitary traceless matrices is presented.

A similar property holds for general quadrics as well as for the circle.