Siu's curvature positivity and $L^2$ extension theorems for $(p,q)$-forms
In this paper, we introduce Siu's curvature operator \(A^E_{p,q}\) for vector-bundle-valued differential forms on K\"ahler manifolds. When $p=n$, this operator reduces to the classical Akizuki--Nakano curvature operator. We first characterize the semipositivity of \(A^E_{p,q}\) in terms of an optimal \(L^2\)-estimate condition for the \(\bar\partial\)-operator, and then prove an Ohsawa--Takegoshi-type extension theorem for \(E\)-valued \((p,q)\)-forms under the curvature condition \(A^E_{p,q+1}\geq0\), using a new twisted basic estimate adapted to this setting. As an application, we prove the local freeness of the higher direct image sheaf \(R^q s_*(\Omega^p_{X/ B_m}\otimes E)\) under the curvature conditions $A^E_{p,q+1}\geq0$ and $A^E_{p,q}\geq0$, where $s: X \to B_m:=\{t\in\mathbb C^m:\ |t|<1\}$ is a proper holomorphic submersion from a K\"ahler manifold $X$, and $E$ is a Hermitian holomorphic vector bundle.