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Search Results: 1 - 10 of 58965 matches for " Xiaokui Yang "
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The Chern-Ricci flow and holomorphic bisectional curvature
Xiaokui Yang
Mathematics , 2015,
Abstract: In this note, we show that on Hopf manifold $\mathbb S^{2n-1}\times \mathbb S^1$, the non-negativity of the holomorphic bisectional curvature is not preserved along the Chern-Ricci flow.
Hermitian manifolds with semi-positive holomorphic sectional curvature
Xiaokui Yang
Mathematics , 2014,
Abstract: We prove that a compact Hermitian manifold with semi-positive but not identically zero holomorphic sectional curvature has Kodaira dimension $-\infty$. As applications, we show that Kodaira surfaces and hyperelliptic surfaces can not admit Hermitian metrics with semi-positive holomorphic sectional curvature although they have nef tangent bundles.
Big vector bundles and compact complex manifolds with semi-positive tangent bundles
Xiaokui Yang
Mathematics , 2014,
Abstract: We classify compact K\"ahler manifolds with semi-positive holomorphic bisectional and big tangent bundles. We also classify compact complex surfaces with semi-positive tangent bundles and compact complex $3$-folds of the form $P(T^*X)$ whose tangent bundles are nef. Moreover, we show that if $X$ is a Fano manifold such that $P(T^*X)$ has nef tangent bundle, then $X\cong P^n$.
Geometry of Hermitian manifolds
Kefeng Liu,Xiaokui Yang
Mathematics , 2010,
Abstract: On Hermitian manifolds, the second Ricci curvature tensors of various metric connections are closely related to the geometry of Hermitian manifolds. By refining the Bochner formulas for any Hermitian complex vector bundle (Riemannain real vector bundle) with an arbitrary metric connection over a compact Hermitian manifold, we can derive various vanishing theorems for Hermitian manifolds and complex vector bundles by the second Ricci curvature tensors. We will also introduce a natural geometric flow on Hermitian manifolds by using the second Ricci curvature tensor.
Effective vanishing theorems for ample and globally generated vector bundles
Kefeng Liu,Xiaokui Yang
Mathematics , 2013,
Abstract: By proving an integral formula of the curvature tensor of $E\ts \det E$, we observe that the curvature tensor of $E\ts \det E$ is very similar to that of a line bundle and obtain certain new Kodaira-Akizuki-Nakano type vanishing theorems for vector bundles. As special cases, we deduce vanishing theorems for ample, nef and globally generated vector bundles by analytic method instead of the Leray-Borel-Le Potier spectral sequence.
Curvatures of direct image sheaves of vector bundles and applications I
Kefeng Liu,Xiaokui Yang
Mathematics , 2013,
Abstract: Let $p:\sXS$ be a proper K\"ahler fibration and $\sE\sX$ a Hermitian holomorphic vector bundle. As motivated by the work of Berndtsson(\cite{Berndtsson09a}), by using basic Hodge theory, we derive several general curvature formulas for the direct image $p_*(K_{\sX/S}\ts \sE)$ for general Hermitian holomorphic vector bundle $\sE$ in a simple way. A straightforward application is that, if the family $\sXS$ is infinitesimally trivial and Hermitian vector bundle $\sE$ is Nakano-negative along the base $S$, then the direct image $p_*(K_{\sX/S}\ts \sE)$ is Nakano-negative. We also use these curvature formulas to study the moduli space of projectively flat vector bundles with positive first Chern classes and obtain that, if the Chern curvature of direct image $p_*(K_{X}\ts E)$--of a positive projectively flat family $(E,h(t))_{t\in \mathbb D}X$--vanishes, then the curvature forms of this family are connected by holomorphic automorphisms of the pair $(X,E)$.
Ricci Curvatures on Hermitian manifolds
Kefeng Liu,Xiaokui Yang
Mathematics , 2014,
Abstract: In this paper, we introduce the first Aeppli-Chern class for complex manifolds and show that the $(1,1)$- component of the curvature $2$-form of the Levi-Civita connection on the anti-canonical line bundle represents this class. We also derive curvature relations on Hermitian manifolds and the background Riemannian manifolds. Moreover, we study non-K\"ahler Calabi-Yau manifolds by using the first Aeppli-Chern class and the Levi-Civita Ricci-flat metrics. In particular, we construct explicit Levi-Civita Ricci-flat metrics on Hopf manifolds $S^{2n-1}\times S^1$. We also construct a smooth family of Gauduchon metrics on a compact Hermitian manifolds such that the metrics are in the same first Aeppli-Chern class, and their first Chern-Ricci curvatures are the same and nonnegative, but their Riemannian scalar curvatures are constant and vary smoothly between negative infinity and a positive number. In particular, we show that Hermitian manifolds with nonnegative first Chern class can admit Hermitian metrics with strictly negative Riemannian scalar curvatures.
Hermitian Harmonic maps and non-degenerate curvatures
Kefeng Liu,Xiaokui Yang
Mathematics , 2014,
Abstract: In this paper, we study the existence of various harmonic maps from Hermitian manifolds to Kaehler, Hermitian and Riemannian manifolds respectively. By using refined Bochner formulas on Hermitian (possibly non-Kaehler) manifolds, we derive new rigidity results on Hermitian harmonic maps from compact Hermitian manifolds to Riemannian manifolds, and we also obtain the complex analyticity of pluri-harmonic maps from compact complex manifolds to compact Kaehler manifolds (and Riemannian manifolds) with non-degenerate curvatures, which are analogous to several fundamental results in [28,Siu], [14,Jost-Yau] and [26, Sampson].
An extension of a theorem of Wu-Yau
Valentino Tosatti,Xiaokui Yang
Mathematics , 2015,
Abstract: We show that a compact Kahler manifold with nonpositive holomorphic sectional curvature has nef canonical bundle. If the holomorphic sectional curvature is negative then it follows that the canonical bundle is ample, confirming a conjecture of Yau. The key ingredient is the recent solution of this conjecture in the projective case by Wu-Yau.
Positivity and vanishing theorems for ample vector bundles
Kefeng Liu,Xiaofeng Sun,Xiaokui Yang
Mathematics , 2010,
Abstract: In this paper, we study the Nakano-positivity and dual-Nakano-positivity of certain adjoint vector bundles associated to ample vector bundles. As applications, we get new vanishing theorems about ample vector bundles. For example, we prove that if $E$ is an ample vector bundle over a compact K\"ahler manifold $X$, $S^kE\ts \det E$ is both Nakano-positive and dual-Nakano-positive for any $k\geq 0$. Moreover, $H^{n,q}(X,S^kE\ts \det E)=H^{q,n}(X,S^kE\ts \det E)=0$ for any $q\geq 1$. In particular, if $(E,h)$ is a Griffiths-positive vector bundle, the naturally induced Hermitian vector bundle $(S^kE\ts \det E, S^kh\ts \det h)$ is both Nakano-positive and dual-Nakano-positive for any $k\geq 0$.
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