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Geometric Genus of Plane Projective Curves
Let C be a plane projective curve. We compute the geometric genus by a local analysis of the singularities.

Choose an affine covering of P2, and assume that in such an open affine set the curve C is defined by f=0.

We know:

  • the ideal   Sing(f):= < f, fx, fy >   defines the singular locus

  • the ideal   Sing(Sing(f)):=< f, fx, fy, det(Hess(f)) >   defines the non-nodal locus

  • the ideal   S:= Sing(Sing(Sing(f)))   defines the non-nodal-cuspidal locus

  • delta(C,x)=1 in nodal or cuspidal singularities, so we just have to count them.

  • the singular points different from cusps and nodes are obtained by a primary decomposition of S.

Note:  The primary decomposition is done over Q. To obtain the points, we have to extend the field. Analysis of Singularities

Sao Carlos, 08/02 http://www.singular.uni-kl.de