Surfaces of Degree 10 in the Projective Fourspace via Linear Systems and Linkage
The paper discusses the classification of surfaces of degree 10 and sectional genus 9 and 10. The surfaces of degree at most 9 are described through classical work dating from the last century up to recent years, while surfaces of degree 10 and other sectional genera are studied elsewhere. We use relations between multisecants, linear systems, syzygies and linkage to describe the geometry of each surface. We want in fact to stress the importance of multisecants and syzygies for the study of these surfaces. Adjunction, which provided efficient arguments for the classification of surfaces of smaller degrees, here appears to be less effective and will play almost no role in the proofs. We show that there are 8 different families of smooth surfaces of degree 10 and sectional genus 9 and 10. The families are determined by numerical data such as the sectional genus, the Euler characteristic, the number of 6-secants to the surface and the number of 5-secants to the surface which meet a general plane. For each type we describe the linear system giving the embedding in P^4, the resolution of the ideal, the geometry of the surface in terms of curves on the surface and hypersurfaces containing the surface, and the liaison class; in particular we find minimal elements in the even liaison class. Each type corresponds to an irreducible, unirational component of the Hilbert scheme, and the dimension is computed.