Special Weierstrass points on algebraic curves in ${\mathbb{P}}^1\times {\mathbb{P}}^1$

Abstract

In this paper we demonstrate that the notion of inflection points and extactic points on plane algebraic curves can be suitably transferred to curves in ${\mathbb{P}}^1\times {\mathbb{P}}^1$. More precisely, we describe osculating curves and study Weierstrass points of algebraic curves in the surface ${\mathbb{P}}^1\times {\mathbb{P}}^1$ with respect to certain linear systems. In particular, we study points where a fiber of ${\mathbb{P}}^1\times {\mathbb{P}}^1$ is tangent, and points with a hyperosculating $(1,1)$-curve. In the first case we find Hessian-like curves that intersect the curve in these points, and in the second case we find a local criteria. Moreover, we provide Plücker-like formulas for the number of smooth Weierstrass points on a curve. In the special case of rational curves, we use suitable Wronskians to compute these points and their respective Weierstrass weights.

18 pages