For example , the Fermat problem of finding sphere ( s ) tangent to four given spheres is a generalization of Apollonius ' problem , whereas Soddy 's hexlet is a generalization of a Steiner chain .
Problems involving tangent circles are often generalized to spheres .
The Fermat problem of finding sphere ( s ) tangent to four given spheres is a generalization of Apollonius ' problem , whereas Soddy 's hexlet is a generalization of a Steiner chain .
Problems involving tangent circles are often generalized to spheres .