Hirzebruch manifolds and positive holomorphic sectional curvature

This paper is the first step in a systematic project to study examples of K"ahler manifolds with positive holomorphic sectional curvature ($H>0$). Previously Hitchin proved that any compact K"ahler surface with $H>0$ must be rational and he constructed such examples on Hirzebruch surfaces $M_{2, k}=$\mathbb{P}$(H^{k}\oplus 1_{$\mathbb{C}\mathbb{P}$^1})$. We generalize Hitchin’s construction and prove that any Hirzebruch manifold $M_{n, k}=$\mathbb{P}$(H^{k}\oplus 1_{$\mathbb{C}\mathbb{P}$^{n-1}})$admitsaK\ddot{a}hlermetricof$H>0$ in each of its K"ahler classes. We demonstrate that the pinching behaviors of holomorphic sectional curvatures of new examples differ from those of Hitchin’s which were studied in the recent work of Alvarez-Chaturvedi-Heier. Some connections to recent works on the K"ahler-Ricci flow on Hirzebruch manifolds are also discussed. It seems interesting to study the space of all K"ahler metrics of $H>0$ on a given K"ahler manifold. We give higher dimensional examples such that some K"ahler classes admit K"ahler metrics with $H>0$ and some do not.