Martin Peternell Boris Odehnal Maria Lucia Sampoli On quadratic two-parameter families of spheres and their envelopes. 342-355 2008 25 Computer Aided Geometric Design 4-5 http://dx.doi.org/10.1016/j.cagd.2007.10.007 db/journals/cagd/cagd25.html#PeternellOS08