intersection divisor for a quartic
Let be a non-singular curve in the plane, defined over an algebraically closed field , and given by a polynomial of degree (i.e. is a quartic). Let be a (rational) line in the plane . The intersection
divisor of and is of the form:
where , , are points in . There are five possibilities:
- 1.
The generic position: all the points are distinct.
- 2.
is tangent to : there exist indices such that . Without loss of generality we may assume and , and .
- 3.
is bitangent to when and but . It may be shown that if then has exactly bitangent lines.
- 4.
intersects at exactly two points, thus . The point is called a flex.
- 5.
intersects at exactly one point and . This point is called a hyperflex. A quartic may not have any hyperflex.
References
- 1 S. Flon, R. Oyono, C. Ritzenthaler, Fast addition
on non-hyperelliptic genus 3 curves, can be found http://eprint.iacr.org/2004/118.pshere.