The picture at left shows the 28 bitangent lines to the red quartic curve with real locus consisting of 4 ovals. Each line is tangent to the curve at 2 points. The bitangents are colored so that rotation by 90 degrees maps a line to one of the same color. This curve is a smooth plane quartic, so it has genus 3. It is a theorem of Harnack that at most g+1 ovals are possible in the real locus of a plane curve. The configuration of lines has automorphism group isomorphic to the Weyl group of the exceptional Lie algebra E7. Every plane projective quartic has 28 bitangents.


Mathematics 536: Introduction to Algebraic Geometry II, Spring 2003


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References and guide to the literature

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