On the classification of toric $2$-Fano manifolds: generic $\mathbb{P}^2$-bundles

Abstract

In this paper, we advance the classification of toric 2-Fano manifolds by continuing the investigation of the minimal projective bundle dimension $m(X) \in \{1,\dots,\dim(X)\}$ introduced in our previous work. This invariant captures the minimal degree of a dominating family of rational curves on $X$ and admits a natural combinatorial interpretation in terms of centered primitive collections. We develop an approach that relates, via toric blowdowns and flips, a toric Fano manifold $X$ to a toric manifold $Y$ that admits a $\mathbb{P}^{m(X)}$-bundle structure on a big open subset. We then compare positivity of the second Chern characters of $X$ and $Y$, and show that the only toric 2-Fano manifold $X$ with $m(X) = 2$ is $X\cong \mathbb{P}^2$. In the example-driven Appendix B, we demonstrate that extending this strategy to the case $m(X)>2$ requires either a substantially more detailed analysis of the combinatorics of primitive collections or a fundamentally new approach.