Quaternionic Hermitian Manifolds Research Articles

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.

On the almost h-conformal semi-invariant Riemannian maps

In this paper, we introduce hsR maps and ahsR maps from almost quaternionic Hermitian manifolds into Riemannian manifolds as a generalization of almost h-conformal semi-invariant submersions and conformal semi-invariant Riemannian maps. We give some examples of these maps and study their properties: the geometry of foliations, the integrability of distributions, the conditions for these maps to be harmonic or totally geodesic.

Almost h-conformal semi-invariant submersions from almost quaternionic Hermitian manifolds

As a generalization of Riemannian submersions, horizontally conformal submersions, semi-invariant submersions, h-semi-invariant submersions, almost h-semi-invariant submersions, conformal semi-invariant submersions, we introduce h-conformal semi-invariant submersions and almost h-conformal semi-invariant submersions from almost quaternionic Hermitian manifolds onto Riemannian manifolds. We study their properties: the geometry of foliations, the conditions for total manifolds to be locally product manifolds, the conditions for such maps to be totally geodesic. Finally, we give some examples of such maps.

Submaximally symmetric quaternion Hermitian structures

We consider and resolve the gap problem for almost quaternion-Hermitian structures, i.e. we determine the maximal and submaximal symmetry dimensions, both for Lie algebras and Lie groups, in the class of almost quaternion-Hermitian manifolds. We classify all structures with such symmetry dimensions. Geometric properties of the submaximally symmetric spaces are studied, in particular, we identify locally conformally quaternion-Kähler structures as well as quaternion-Kähler with torsion.

English

We introduce the notions of h-conformal anti-invariant submersions and h-conformal Lagrangian submersions from almost quaternionic Hermitian manifolds onto Riemannian manifolds as a generalization of Riemannian submersions, horizontally conformal submersions, anti-invariant submersions, h-anti-invariant submersions, h-Lagrangian submersion, conformal anti-invariant submersions. We investigate their properties: the integrability of distributions, the geometry of foliations, the conditions for such maps to be totally geodesic, etc. Finally, we give some examples of such maps.

Almost h-conformal slant submersions from almost quaternionic Hermitian manifolds

We introduce the notions of h-conformal slant submersions and almost h-conformal slant submersions from almost quaternionic Hermitian manifolds onto Riemannian manifolds as a generalization of Riemannian submersions, horizontally conformal submersions, slant submersions, h-slant submersions, almost h-slant submersion and conformal slant submersions. We investigate several properties of these including the integrability of distributions, the geometry of foliations and the conditions for such maps to be totally geodesic. Further we give some examples of such maps.

English

As a generalization of anti-invariant Riemannian submersions and Lagrangian Riemannian submersions, we introduce the notions of h-anti-invariant submersions and h-Lagrangian submersions from almost quaternionic Hermitian manifolds onto Riemannian manifolds. We obtain characterizations and investigate some properties: the integrability of distributions, the geometry of foliations, and the harmonicity of such maps. We also find a condition for such maps to be totally geodesic and give some examples of such maps. Finally, we obtain some types of decomposition theorems.

Twistor spaces of Riemannian manifolds with even Clifford structures

In this paper, we introduce the twistor space of a Riemannian manifold with an even Clifford structure. This notion generalizes the twistor space of quaternion-Hermitian manifolds and weak- $$\mathrm {Spin}(9)$$ structures. We also construct almost complex structures on the twistor space for parallel even Clifford structures and check their integrability. Moreover, we prove that in some cases one can give Kahler and nearly Kahler metrics to these spaces.

Almost h-semi-slant Riemannian maps to almost quaternionic Hermitian manifolds

We introduce the notions of almost h-slant Riemannian maps, almost h-semi-invariant Riemannian maps, and almost h-semi-slant Riemannian maps from Riemannian manifolds to almost quaternionic Hermitian manifolds. We investigate the harmonicity of such maps and the geometry of distributions. We also find the conditions for such maps to be totally geodesic, relate the notion of pseudo-horizontally weakly conformal maps to those notions, and give some examples of such maps.

$H$-SEMI-SLANT SUBMERSIONS FROM ALMOST QUATERNIONIC HERMITIAN MANIFOLDS

As a generalization of semi-slant submersions, h-slant submersions, and h-semi-invariant submersions, we introduce the notions of h-semi-slant submersions and almost h-semi-slant submersions from almost quaternionic Hermitian manifolds onto Riemannian manifolds. We obtain characterizations and investigate the integrability of distributions, the geometry of fibers, and the harmonicity of such maps. We also find a condition for such maps to be totally geodesic. Moreover, we give some examples of such maps.

ALMOST $h$-SEMI-SLANT RIEMANNIAN MAPS

As a generalization of slant Riemannian maps, semi-slant Riemannian maps, almost h-slant submersions, and almost h-semi-slant submersions, we introduce the notion of almost h-semi-slant Riemannian maps from almost quaternionic Hermitian manifolds to Riemannian manifolds. We investigate the integrability of distributions, the harmonicity of such maps, the geometry of fibers, etc. We also deal with the condition for such maps to be totally geodesic and study some decomposition theorems. Moreover, we give some examples.

Homogeneous almost quaternion-Hermitian manifolds

An almost quaternion-Hermitian structure on a Riemannian manifold $(M^{4n},g)$ is a reduction of the structure group of $M$ to $\mathrm{Sp}(n)\mathrm{Sp}(1)\subset \mathrm{SO}(4n)$. In this paper we show that a compact simply connected homogeneous almost quaternion-Hermitian manifold of non-vanishing Euler characteristic is either a Wolf space, or $\mathbb{S}^2\times \mathbb{S}^2$, or the complex quadric $\mathrm{SO}(7)/\mathrm{U}(3)$.

Quaternionic Kähler and Spin(7) metrics arising from quaternionic contact Einstein structures

We construct left-invariant quaternionic contact (qc) structures on Lie groups with zero and non-zero torsion and with non-vanishing quaternionic contact conformal curvature tensor, thus showing the existence of non-flat quaternionic contact manifolds. We prove that the product of the real line with a seven-dimensional manifold, equipped with a certain qc structure, has a quaternionic Kahler metric as well as a metric with holonomy contained in Spin(7). As a consequence, we determine explicit quaternionic Kahler metrics and Spin(7)-holonomy metrics, which seem to be new. Moreover, we give explicit non-compact eight dimensional almost quaternion hermitian manifolds that are not quaternionic Kahler with either a closed fundamental four form or fundamental two forms defining a differential ideal.

Dirichlet Duality and the Nonlinear Dirichlet Problem on Riemanninan Manifolds

In this paper we study the Dirichlet problem for fully nonlinear second-order equations on a riemannian manifold. As in our previous paper, "Dirichlet duality and the non-linear Dirichlet problem," Comm. on Pure and Applied Math., 62 (2009), 396–443, we define equations via closed subsets of the 2-jet bundle where each equation has a natural dual equation. Basic existence and uniqueness theorems are established in a wide variety of settings. However, the emphasis is on starting with a constant coefficient equation as a model, which then universally determines an equation on every riemannian manifold which is equipped with a topological reduction of the structure group to the invariance group of the model. For example, this covers all branches of the homogeneous complex Monge-Ampère equation on an almost complex hermitian manifold $X$. In general, for an equation $F$ on a manifold $X$ and a smooth domain $\Omega \subset\subset X$, we give geometric conditions which imply that the Dirichlet problem on $\Omega$ is uniquely solvable for all continuous boundary functions. We begin by introducing a weakened form of comparison which has the advantage that local implies global. We then introduce two fundamental concepts. The first is the notion of a monotonicity cone $M$ for $F$. If $X$ carries a global $M$-subharmonic function, then weak comparison implies full comparison. The second notion is that of boundary $F$-convexity, which is defined in terms of the asymptotics of $F$ and is used to define barriers. In combining these notions the Dirichlet problem becomes uniquely solvable as claimed. This article also introduces the notion of local affine jet-equivalence for subequations. It is used in treating the cases above, but gives results for a much broader spectrum of equations on manifolds, including inhomogeneous equations and the Calabi-Yau equation on almost complex hermitian manifolds. A considerable portion of the paper is concerned with specific examples. They include a wide variety of equations which make sense on any riemannian manifold, and many which hold universally on almost complex or quaternionic hermitian manifolds, or topologically calibrated manifolds.

Almost quaternion-hermitian manifolds with large automorphism group

We study almost quaternion-hermitian manifolds with large automorphism group, and classify them when the dimension of the group is close to the maximal dimension.

Curvature of almost quaternion-Hermitian manifolds

We study the decomposition of the Riemannian curvature R tensor of an almost quaternion-Hermitian manifold under the action of its structure group Sp( n ) Sp(1). Using the minimal connection, we show that most components are determined by the intrinsic torsion ξ and its covariant derivative and determine relations between the decompositions of ξ ⊗ ξ , and R . We pay particular attention to the behaviour of the Ricci curvature and the q-Ricci curvature.

The intrinsic torsion of almost quaternion-Hermitian manifolds

We study the intrinsic torsion of almost quaternion-Hermitian manifolds via the exterior algebra. In particular, we show how it is determined by particular three-forms formed from simple combinations of the exterior derivatives of the local Kähler forms. This gives a practical method to compute the intrinsic torsion and is applied in a number of examples. In addition we find simple characterisations of HKT and QKT geometries entirely in the exterior algebra and compute how the intrinsic torsion changes under a twist construction.

Harmonic maps between quaternionic Kähler manifolds

In this note we introduce the concept of (σ, σ′)-holomorphic map between two almost quaternionic Hermitian manifolds. We prove that a (σ, σ′)-holomorphic map between two quaternionic Kähler manifolds with a certain property is a harmonic map and give some conditions for the stability of such a map.