Non-free curves on Fano varieties

Abstract

Let $X$ be a smooth Fano variety over $\mathbb{C}$ and let $B$ be a smooth projective curve over $\mathbb{C}$. Geometric Manin's Conjecture predicts the structure of the irreducible components $M\subset \mathrm{Mor}(B,X)$ parametrizing curves which are non-free and have large anticanonical degree. Following ideas of our previous work, we prove the first prediction of Geometric Manin's Conjecture describing such irreducible components. As an application, we prove that there is a proper closed subset $V\subset X$ such that all non-dominant components of $\mathrm{Mor}(B,X)$ parametrize curves in $V$, verifying an expectation put forward by Victor Batyrev. We also demonstrate two important ways that studying $\mathrm{Mor}(B,X)$ differs from studying the space of sections of a Fano fibration $\mathcal{X}\to B$.

minor revisions, 31 pages, to appear in Osaka J. of Math.,. arXiv admin note: text overlap with arXiv:2301.01695