On regular vertices on the union of planar objects.

Esther Ezra János Pach Micha Sharir On regular vertices on the union of planar objects. 220-226 2007 conf/compgeom/2007 Symposium on Computational Geometry http://doi.acm.org/10.1145/1247069.1247110 db/conf/compgeom/compgeom2007.html#EzraPS07