Weierstrass points on certain non-classical curves

Archiv der Mathematik -     

Weierstrass points and multiple intersection points of closed geodesics

We classify smooth complex projective surfaces of degreed and class , satisfying either (i) –d16, or (ii) 25. All these surfaces are rational or ruled. Indeed, we prove that the smallest value of the class of a non-ruled surface is 30 and in fact there are at least two surfacesS, both of degreed=10 and sectional genusg=6, with Kodaira dimension (S)=0 and class =30. Finally, we classify the smoothk-folds (k3) whose sectional surface has class 23.     

Families of special Weierstrass points

The purpose of this Note is to show that loci of (special) Weierstrass points on the fibers of a family $\pi :\mathfrak{X}\to S$ of smooth curves of genus $g?2$ can be studied by simply pulling back the Schubert calculus naturally living on a suitable Grassmann bundle over $\mathfrak{X}$. Using such an idea we prove new results regarding the decomposition in $A_1(\mathfrak{X})$ of the class of the locus of Weierstrass points having weight at least 3 as the sum of classes of Weierstrass points having “bounded from below” gaps sequences. To cite this article: L. Gatto, P. Salehyan, C. R. Acad. Sci. Paris, Ser. I 347 (2009).     

The monodromy of Weierstrass points

Summary We show that the monodromy of the family of curves (Riemann surfaces) acts as the full symmetric group on the Weierstrass points of a general curve. The proof uses a degeneration to certain reducible curves, and the theory of limit series developed in our (1986, 1987a, b). Some of the monodromy is actually constructed by fixing a (reducible) curve and varying its canonical series.Both authors are grateful to the National Science Foundation for partial support during the preparation of this work     

Weierstrass points and Z2 homology

In a recent paper (1993), Lustig established a beautiful connection between the six Weierstrass points on a Riemann surface M₂ of genus 2 and intersection points of closed geodesics for the associated hyperbolic metric. As a consequence, he was able to construct an action of the mapping class group Out(π₁M₂) of M₂ on the set of Weierstrass points of M₂ and a virtual splitting of the natural homomorphism Aut(π₁M₂) → Out(π₁M₂). Our discussion in this paper begins with the observation that these two results of Lustig's are direct consequences of the work of Birman and Hilden (1973) on equivariant homotopies for surface homeomorphisms.It is well known that Γ₂ acts naturally on the Z₂ symplectic vector space of rank 4, H₁(M₂, Z₂). We identify this action with Lustig's action by constructing a natural correspondence between pairs of distinct Weierstrass points on M₂ and nonzero elements in H₁(M₂,Z₂). In this manner, the well-known exceptional isomorphism of finite group theory, S₆ ≅ Sp(4, Z₂), arises from a natural isomorphism of Γ₂ spaces.     

Ramification points of double coverings of curves and Weierstrass points

Summary Let π: X→C be a double covering with X smooth curve and C elliptic curve. Let R(π)⊂X be the ramification locus of π. Every P∈R(π) is a Weierstrass point of X and we study the triples (C, π, X) for which the set of corresponding Weierstrass points have certain semigroups of non-gaps. We study the same problem also for triple cyclic coverings of C. Entrata in Redazione il 17 luglio 1998. The authors were partially supported by MURST and GNSAGA of CNR (Italy).     

On the constellations of Weierstrass points

We prove that the constellation of Weierstrass points characterizes the isomorphism-class of double coverings of curves of genus large enough. The author was supported by a grant from the International Atomic Energy Agency and UNESCO.     

Weierstrass multiple points and ramification points of smooth projective curves

Summary In this paper we study finite sets of smooth algebraic curves which are the support of special divisors («Weierstrass sets»). We prove several existence results of Weierstrass sets with low weight on suitable curves (e.g. general k-gonal curves).     

Local invariants attached to Weierstrass points

Let X/S be a hyperelliptic curve of genus g over the spectrum of a discrete valuation ring. Two fundamental numerical invariants are attached to X/S: the valuation d of the hyperelliptic discriminant of X/S, and the valuation δ of the Mumford discriminant of X/S (equivalently, the Artin conductor). For a residue field of characteristic 0 as well as for X/S semistable the invariants d and δ are known to satisfy certain inequalities. We prove an exact formula relating d and δ with intersection theoretic data determined by the distribution of Weierstrass points over the special fiber, in the semistable case. We also prove an exact formula for the stable Faltings height of an arbitrary curve over a number field, involving local contributions associated to its Weierstrass points.     

Weierstrass points on trigonal curves I: The ramification points

LetC be a smooth curve of genusg≥5. Assume thatP is a Weierstrass point onC which first non-gap is equal to 3. The gap sequence atP is completely determinated by numbersn and ε satisfying (g−1)/3≤n≤g/2 and ε is 1 or 2 as follows. Given suchn and ε, the corresponding gap sequence is (1, 2, 4, 5,…, 3n−2, 3n−1, 3n+ε, 3n+3+ε, …, 3(g−n−1)+ε). We say thatP is of then-th kind andP is of type I (resp. II) if ε=1 (resp. 2). Because a curve of genusg≥5 has at most one linear systemg1/3, it follows that the Weierstrass points onC with first non-gap equal to 3 are of the same kind.