Except for triangles, every simple polygon has at least two nonoverlapping ears.
de Berg, M.; van Kreveld, M.; Overmans, M.; and Schwarzkopf, O. Computational Geometry: Algorithms and Applications, 2nd rev. ed. Berlin: Springer-Verlag, p. 59, 2000.
Meisters, G. H. "Principal Vertices, Exposed Points, and Ears." Amer. Math. Monthly 87, 284-285, 1980.
Toussaint, G. "Anthropomorphic Polygons." Amer. Math. Monthly 122, 31-35, 1991.
Weisstein, Eric W. "Two-Ears Theorem." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/Two-EarsTheorem.html
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