# Dupin cyclide

Image credit: Robert March.

A Dupin cyclide is the envelope of all spheres touching three
given fixed spheres. (Each of the fixed spheres is to be touched in an assigned
manner, either externally or internally.) Equivalently, it is the envelope of
all spheres whose centers lie on a given conic
section and which touch a given sphere. Also equivalently, it is the inverse
of a torus.