Hermitian Manifolds Research Articles

$\overline{\partial}$-Harmonic forms on $4$-dimensional almost-Hermitian manifolds

$\overline{\partial}$-Harmonic forms on $4$-dimensional almost-Hermitian manifolds

Hodge modules and singular hermitian metrics

The purpose of this paper is to study certain notions of metric positivity called “minimal extension property” for the lowest nonzero piece in the Hodge filtration of a Hodge module. Let X be a complex manifold and let \(\mathcal {M}\) be a polarized pure Hodge module on X with strict support X. Let \(F_p\mathcal {M}\) be the smallest nonzero piece in the Hodge filtration. Assume that \(\mathcal {M}\) is smooth outside a closed analytic subset Z and let \(j:X\setminus Z \hookrightarrow X\) be the open embedding. Let h be the smooth hermitian metric on \(F_p\mathcal {M}|_{X\setminus Z}\) induced by the polarization. We show that the canonical morphism of \(\mathcal {O}_X\)-modules $$\begin{aligned} F_p\mathcal {M}\rightarrow j_{*}(F_p\mathcal {M}|_{X\setminus Z}) \end{aligned}$$induces an isomorphism between \(F_p\mathcal {M}\) and the subsheaf of \(j_{*}(F_p\mathcal {M}|_{X\setminus Z})\) consisting of sections which are locally \(L^2\) near Z with respect to h and the standard Lebesgue measure on X. In particular, h extends to a singular hermitian metric on \(F_p\mathcal {M}\) with minimal extension property.

Canonical quasi-geodesic mappings of special pseudo-Riemannian spaces

The present paper continues the study of quasi-geodesic mappings f:(Vn, gij, Fih) → (V'n,g'ij, Fih) of pseudo-Riemannian spaces Vn, V'n with a generalized-recurrent structure Fih of parabolic type. By a generalized recurrent structure of parabolic type on Vn we mean an almost Hermitian affinor structure of parabolic type for which the covariant derivative of the structural affinor Fih satisfies the condition F(i,j)h=q(i Fj)h.
 In the previous paper by the authors [Proc. Intern. Geom. Center, 13:3 (2020) 18-32] it was proved that the class of pseudo-Riemannian spaces with generalized-recurrent structure of parabolic type is closed with respect to the considered mappings and the generalized recurrence vectors in (Vn, gij,Fih) and (V'_n, g'ij, Fih) may be distinct. In this article, it is assumed that the mapping f preserves the generalized recurrence vector qi.
 We construct geometric objects that are invariant under the quasi-geodesic mapping of generalized-recurrent spaces of parabolic type and recurrent-parabolic spaces. A number of conditions are given on these objects, which lead to the fact that a generalized-recurrent space of parabolic type admits a parabolic K-structure, and a recurrent-parabolic space admits a Kählerian structure of parabolic type.
 We study special types of these mappings that preserve some tensors of an intrinsic nature.

Locally conformal SKT structures

A Hermitian metric on a complex manifold is called SKT (strong Kähler with torsion) if the Bismut torsion 3-form [Formula: see text] is closed. As the conformal generalization of the SKT condition, we introduce a new type of Hermitian structure, called locally conformal SKT (or shortly LCSKT). More precisely, a Hermitian structure [Formula: see text] is said to be LCSKT if there exists a closed nonzero [Formula: see text]-form [Formula: see text] such that [Formula: see text]. In this paper, we consider nontrivial LCSKT structures, i.e. we assume that [Formula: see text] and we study their existence on Lie groups and their compact quotients by lattices. In particular, we classify six-dimensional nilpotent Lie algebras admitting a LCSKT structure and we show that, in contrast to the SKT case, there exists a six-dimensional 3-step nilpotent Lie algebra admitting a nontrivial LCSKT structure. Moreover, we show a characterization of even dimensional almost abelian Lie algebras admitting a nontrivial LCSKT structure, which allows us to construct explicit examples of six-dimensional unimodular almost abelian Lie algebras admitting a nontrivial LCSKT structure. The compatibility between the LCSKT and the balanced condition is also discussed, showing that a Hermitian structure on a six-dimensional nilpotent or a [Formula: see text]-dimensional almost abelian Lie algebra cannot be simultaneously LCSKT and balanced, unless it is Kähler.

Geometric Invariants for a Class of Submodules of Analytic Hilbert Modules Via the Sheaf Model

Let \(\Omega \subseteq {\mathbb {C}}^m\) be a bounded connected open set and \({\mathcal {H}} \subseteq {\mathcal {O}}(\Omega )\) be an analytic Hilbert module, i.e., the Hilbert space \({\mathcal {H}}\) possesses a reproducing kernel K, the polynomial ring \(\mathbb C[{\varvec{z}}]\subseteq {\mathcal {H}}\) is dense and the point-wise multiplication induced by \(p\in {\mathbb {C}}[{\varvec{z}}]\) is bounded on \({\mathcal {H}}\). We fix an ideal \({\mathcal {I}} \subseteq {\mathbb {C}}[{\varvec{z}}]\) generated by \(p_1,\ldots ,p_t\) and let \([{\mathcal {I}}]\) denote the completion of \({\mathcal {I}}\) in \(\mathcal H\). The sheaf \({\mathcal {S}}^{\mathcal {H}}\) associated to analytic Hilbert module \({\mathcal {H}}\) is the sheaf \({\mathcal {O}}(\Omega )\) of holomorphic functions on \(\Omega \) and hence is free. However, the subsheaf \({\mathcal {S}}^{\mathcal [{\mathcal {I}}]}\) associated to \([{\mathcal {I}}]\) is coherent and not necessarily locally free. Building on the earlier work of Biswas, Misra and Putinar (Journal fr die reine und angewandte Mathematik (Crelles Journal) 662:165–204, 2012), we prescribe a hermitian structure for a coherent sheaf and use it to find tractable invariants. Moreover, we prove that if the zero set \(V_{[{\mathcal {I}}]}\) is a submanifold of codimension t, then there is a unique local decomposition for the kernel \(K_{[{\mathcal {I}}]}\) along the zero set that serves as a holomorphic frame for a vector bundle on \(V_{[{\mathcal {I}}]}\). The complex geometric invariants of this vector bundle are also unitary invariants for the submodule \([{\mathcal {I}}] \subseteq {\mathcal {H}}\).

On h-Quasi-Hemi-Slant Riemannian Maps

In the present article, we indroduce and study h-quasi-hemi-slant (in short, h-qhs) Riemannian maps and almost h-qhs Riemannian maps from almost quaternionic Hermitian manifolds to Riemannian manifolds. We investigate some fundamental results mainly on h-qhs Riemannian maps: the integrability of distributions, geometry of foliations, the condition for such maps to be totally geodesic, etc. At the end of this article, we give two non-trivial examples of this notion.

The Dirichlet problem for the Monge–Ampère equation on Hermitian manifolds with boundary

We study weak quasi-plurisubharmonic solutions to the Dirichlet problem for the complex Monge–Ampère equation on a general Hermitian manifold with non-empty boundary. We prove optimal subsolution theorems: for bounded and Hölder continuous quasi-plurisubharmonic functions. The continuity of the solution is proved for measures that are well dominated by capacity, for example measures with L^p, p>1 densities, or moderate measures in the sense of Dinh–Nguyen–Sibony.

Weak Solutions to Monge–Ampère Type Equations on Compact Hermitian Manifold with Boundary

We prove the bounded subsolution theorem for the complex Monge–Ampère type equation, with the right-hand side being a positive Radon measure, on a compact Hermitian manifold with boundary.

Real semisimple Lie groups and balanced metrics

Given any non-compact real simple Lie group \mathrm{G}_o of inner type and even dimension, we prove the existence of an invariant complex structure \mathrm{J} and a Hermitian balanced metric on \mathrm{G}_o and on any compact quotient \mathrm{M}= \Gamma\backslash\mathrm{G}_o , with \Gamma a cocompact lattice. We also prove that (\mathrm{M},\mathrm{J}) does not carry any pluriclosed metric, in contrast to the case of even dimensional compact Lie groups, which admit pluriclosed but not balanced metrics.

Hessian equations of Krylov type on compact Hermitian manifolds

Abstract In this article, we are concerned with the equations of Krylov type on compact Hermitian manifolds, which are in the form of the linear combinations of the elementary symmetric functions of a Hermitian matrix. Under the assumption of the 𝒞-subsolution, we obtain a priori estimates in Γ k − 1 {\Gamma }_{k-1} cone. By using the method of continuity, we prove an existence theorem, which generalizes the relevant results. As an application, we give an alternative way to solve the deformed Hermitian Yang-Mills equation on compact Kähler threefold.

Born-Padé approach to electromagnetic scattering in complex one-dimensional inhomogeneous slabs

Perturbation theory is applied to one-dimensional scattering systems consisting of a general class of inhomogeneous and isotropic slabs having size $L$ described by the relative permittivity $\varepsilon(z) = 1 + \alpha \chi(z)$, where $\chi(z)$ is the electric susceptibility and $\alpha$ the perturbation parameter. The transmitted and reflected amplitudes are shown to be written as Born series in powers of $\alpha$ and Pad\'e approximants are used to obtain analytical results with a high and arbitrary degree of accuracy. The approach is very general and can handle oblique incidence. Examples are given for the transmission and reflection amplitudes of plane waves interacting with Hermitian and non-Hermitian structures with known and unknown exact solutions.

On quasi-geodesic mappings of special pseudo-Riemannian spaces

The present paper continues the study of quasi-geodesic mappings f:(Vn, gij, Fih) → (V'n,g'ij, Fih) of pseudo-Riemannian spaces Vn, V'n with a generalized-recurrent structure Fih of parabolic type. By a generalized recurrent structure of parabolic type on Vn we mean an almost Hermitian affinor structure of parabolic type for which the covariant derivative of the structural affinor Fih satisfies the condition F(i,j)h=q(i Fj)h.
 In the previous paper by the authors [Proc. Intern. Geom. Center, 13:3 (2020) 18-32] it was proved that the class of pseudo-Riemannian spaces with generalized-recurrent structure of parabolic type is closed with respect to the considered mappings and the generalized recurrence vectors in (Vn, gij,Fih) and (V'_n, g'ij, Fih) may be distinct. In this article, it is assumed that the mapping f preserves the generalized recurrence vector qi.
 We construct geometric objects that are invariant under the quasi-geodesic mapping of generalized-recurrent spaces of parabolic type and recurrent-parabolic spaces. A number of conditions are given on these objects, which lead to the fact that a generalized-recurrent space of parabolic type admits a parabolic K-structure, and a recurrent-parabolic space admits a Kählerian structure of parabolic type.
 We study special types of these mappings that preserve some tensors of an intrinsic nature.

Regularity of degenerate k-Hessian equations on closed Hermitian manifolds

Abstract In this article, we are concerned with the existence of weak C 1 , 1 {C}^{1,1} solution of the k k -Hessian equation on a closed Hermitian manifold under the optimal assumption of the function in the right-hand side of the equation. The key points are to show the weak C 1 , 1 {C}^{1,1} estimates. We prove a Cherrier-type inequality to obtain the C 0 {C}^{0} estimate, and the complex Hessian estimate is proved by using an auxiliary function, which was motivated by Hou et al. and Tosatti and Weinkove. Our result generalizes the Kähler case proved by Dinew et al.

Monge—Ampère Type Equations on Almost Hermitian Manifolds

In this paper, we consider the Monge-Amp\`{e}re type equations on compact almost Hermitian manifolds. We derive $C^{\infty}$ a priori estimates under the existence of an admissible $\mathcal{C}$-subsolution. Finally, we obtain an existence result if there exists an admissible supersolution.

Almost Kaehlerian and Hermitian Structures on Four Dimensional Indecomposable Lie Algebras

It is known that from a given almost Hermitian structure on a simply connected Liegroup, one can obtain left-invariant almost Hermitian structure on its Lie algebra.In this work, we consider Mubarakzyanov’s classification of four-dimensional realLie algebras and evaluate the existence of almost Hermitian structures on four dimensional decomposable real Lie algebras. In particular, we focus on almost Kaehlerian and Hermitian structures on these Lie algebras.

Invariant Generalized Almost Complex Structures on Real Flag Manifolds

We characterize those real flag manifolds that can be endowed with invariant generalized almost complex structures. We show that no $GM_2$-maximal real flag manifolds admit integrable invariant generalized almost complex structures. We give a concrete description of the generalized complex geometry on the maximal real flags of type $B_2$, $G_2$, $A_3$, and $D_l$ with $l\geq 5$, where we prove that the space of invariant generalized almost complex structures under invariant $B$-transformations is homotopy equivalent to a torus and we classify all invariant generalized almost Hermitian structures on them.

A metric analogue of Hartogs’ theorem

In this paper we prove a metric version of Hartogs’ theorem where the holomorphic function is replaced by a locally symmetric Hermitian metric. As an application, we prove that if the Kobayashi metric on a strongly pseudoconvex domain with \({\mathcal {C}}^2\) smooth boundary is a Kähler metric, then the universal cover of the domain is the unit ball.

Examples of K-unstable Fano manifolds

We examine various examples of horosymmetric manifolds which exhibit interesting properties with respect to canonical metrics. In particular, we determine when the blow-up of a quadric along a linear subquadric admits K\"ahler-Einstein metrics, providing infinitely many examples of manifolds with no K\"ahler-Ricci solitons that are not K-semistable. Using a different construction, we provide an infinite family of Fano manifolds with no K\"ahler-Einstein metrics but which admit coupled K\"ahler-Einstein metrics. Finally, we elaborate on the relationship between K\"ahler-Ricci solitons and the more general concept of multiplier Hermitian structures and illustrate this with examples related to the two previous families.

Sasakian Structures on Tangent Sphere Bundles of Compact Rank-One Symmetric Spaces

A positive answer is given to the existence of Sasakian structures on the tangent sphere bundle of some Riemannian manifold whose sectional curvature is not constant. Among other results, it is proved that the tangent sphere bundle T_{r}(G/K), for any r> 0, of a compact rank-one symmetric space G/K, not necessarily of constant sectional curvature, admits a unique G-invariant K-contact structure whose characteristic vector field is the standard field of T(G/K). Such a structure is in fact Sasakian and it can be expressed as an induced structure from an almost Hermitian structure on the punctured tangent bundle T(G/K){setminus } {text{ zero } text{ section }}.

Curvature of the base manifold of a Monge–Ampère fibration and its existence

In this paper, we consider a special relative Kähler fibration that satisfies a homogenous Monge–Ampère equation, which is called a Monge–Ampère fibration. There exist two canonical types of generalized Weil–Petersson metrics on the base complex manifold of the fibration. For the second generalized Weil–Petersson metric, we obtain an explicit curvature formula and prove that the holomorphic bisectional curvature is non-positive, the holomorphic sectional curvature, the Ricci curvature, and the scalar curvature are all bounded from above by a negative constant. For a holomorphic vector bundle over a compact Kähler manifold, we prove that it admits a projectively flat Hermitian structure if and only if the associated projective bundle fibration is a Monge–Ampère fibration. In general, we can prove that a relative Kähler fibration is Monge–Ampère if and only if an associated infinite rank Higgs bundle is Higgs-flat. We also discuss some typical examples of Monge–Ampère fibrations.