Otfried Cheong Xavier Goaoc Andreas Holmsen Sylvain Petitjean Helly-Type Theorems for Line Transversals to Disjoint Unit Balls. 194-212 2008 39 Discrete & Computational Geometry 1-3 http://dx.doi.org/10.1007/s00454-007-9022-1 db/journals/dcg/dcg39.html#CheongGHP08