Zvonko Cerin, On butterflies inscribed in a quadrilateral,
Forum Geometricorum, 6 (2006) 241--246.


Abstract:  We explore a configuration consisting of two quadrilaterals which share the intersection of diagonals. We prove results analogous to the Sidney Kung's Butterfly Theorem for Quadrilaterals  in  Kung, Mathematics Magazine, 78 (2005) 314--316.

[ps file][pdf]