# On covering and quasi-unsplit families of curves

### Laurent Bonavero

Université Grenoble I, Saint-Martin-d'Hères, France### Stéphane Druel

Université Grenoble I, Saint-Martin-d'Hères, France### Cinzia Casagrande

Università di Pisa, Italy

## Abstract

Given a covering family $V$ of effective 1-cycles on a complex projective variety $X$, we find conditions allowing to construct a geometric quotient $q~: X \to Y$, with $q$ regular on the whole of $X$, such that every fiber of $q$ is an equivalence class for the equivalence relation naturally defined by $V$. Among others, we show that on a normal and $\Q$-factorial projective variety $X$ with $\dim(X) \leq 4$, every covering and quasi-unsplit family $V$ of rational curves generates a geometric extremal ray of the Mori cone $\overline{\rm NE}(X)$ of classes of effective 1-cycles and that the associated Mori contraction yields a geometric quotient for $V$ provided $X$ has canonical singularities.

## Cite this article

Laurent Bonavero, Stéphane Druel, Cinzia Casagrande, On covering and quasi-unsplit families of curves. J. Eur. Math. Soc. 9 (2007), no. 1, pp. 45–57

DOI 10.4171/JEMS/71