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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.
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