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On Special Berwald Metrics  [cached]
Akbar Tayebi,Esmaeil Peyghan
Symmetry, Integrability and Geometry : Methods and Applications , 2010,
Abstract: In this paper, we study a class of Finsler metrics which contains the class of Berwald metrics as a special case. We prove that every Finsler metric in this class is a generalized Douglas-Weyl metric. Then we study isotropic flag curvature Finsler metrics in this class. Finally we show that on this class of Finsler metrics, the notion of Landsberg and weakly Landsberg curvature are equivalent.
On Special Berwald Metrics  [PDF]
Akbar Tayebi,Esmaeil Peyghan
Mathematics , 2010, DOI: 10.3842/SIGMA.2010.008
Abstract: In this paper, we study a class of Finsler metrics which contains the class of Berwald metrics as a special case. We prove that every Finsler metric in this class is a generalized Douglas-Weyl metric. Then we study isotropic flag curvature Finsler metrics in this class. Finally we show that on this class of Finsler metrics, the notion of Landsberg and weakly Landsberg curvature are equivalent.
Metrics of Special Curvature with Symmetry  [PDF]
Brandon Dammerman
Mathematics , 2006,
Abstract: Various curvature conditions are studied on metrics admitting a symmetry group. We begin by examining a method of diagonalizing cohomogeneity-one Einstein manifolds and determine when this method can and cannot be used. Examples, including the well-known Stenzel metrics, are discussed. Next, we present a simplification of the Einstein condition on a compact four manifold with $T^{2}$-isometry to a system of second-order elliptic equations in two-variables with well-defined boundary conditions. We then study the Einstein and extremal Kahler conditions on Kahler toric manifolds. After constructing explicitly new extremal Kahler and constant scalar curvature metrics, we demonstrate how these metrics can be obtained by continuously deforming the Fubini-Study metric on complex projective space in dimension three. We also define a generalization of Kahler toric manifolds, which we call fiberwise Kahler toric manifolds, and construct new explicit extremal Kahler and constant scalar curvature metrics on both compact and non-compact manifolds in all even dimensions. We also calculate the Futaki invariant on manifolds of this type. After describing an Hermitian non-Kahler analogue to fiberwise Kahler toric geometry, we construct constant scalar curvature Hermitian metrics with $J$-invariant Riemannian tensor. In dimension four, we write down explicitly new constant scalar curvature Hermitian metrics with $J$-invariant Ricci tensor. Finally, we integrate the scalar curvature equation on a large class of cohomogeneity-one metrics.
Special biconformal changes of K?hler surface metrics  [PDF]
Andrzej Derdzinski
Mathematics , 2011, DOI: 10.1007/s00605-011-0345-x
Abstract: The term "special biconformal change" refers, basically, to the situation where a given nontrivial real-holomorphic vector field on a complex manifold is a gradient relative to two K\"ahler metrics, and, simultaneously, an eigenvector of one of the metrics treated, with the aid of the other, as an endomorphism of the tangent bundle. A special biconformal change is called nontrivial if the two metrics are not each other's constant multiples. For instance, according to a 1995 result of LeBrun, a nontrivial special biconformal change exists for the conformally-Einstein K\"ahler metric on the two-point blow-up of the complex projective plane, recently discovered by Chen, LeBrun and Weber; the real-holomorphic vector field involved is the gradient of its scalar curvature. The present paper establishes the existence of nontrivial special biconformal changes for some canonical metrics on Del Pezzo surfaces, viz. K\"ahler-Einstein metrics (when a nontrivial holomorphic vector field exists), non-Einstein K\"ahler-Ricci solitons, and K\"ahler metrics admitting nonconstant Killing potentials with geodesic gradients.
Special Hermitian metrics on compact solvmanifolds  [PDF]
Anna Fino,Luigi Vezzoni
Mathematics , 2015, DOI: 10.1016/j.geomphys.2014.12.010
Abstract: We review some constructions and properties of complex manifolds admitting pluriclosed and balanced metrics. We prove that for a 6-dimensional solvmanifold endowed with an invariant complex structure J having holomorphically trivial canonical bundle the pluriclosed flow has a long time solution for every invariant initial datum. Moreover, we state a new conjecture about the existence of balanced and SKT metrics on compact complex manifolds. We show that the conjecture is true for nilmanifolds of dimension 6 and 8 and for 6-dimensional solvmanifolds with holomorphically trivial canonical bundle.
Supersymmetric Domain Walls from Metrics of Special Holonomy  [PDF]
G. W. Gibbons,H. Lu,C. N. Pope,K. S. Stelle
Physics , 2001, DOI: 10.1016/S0550-3213(01)00640-X
Abstract: Supersymmetric domain-wall spacetimes that lift to Ricci-flat solutions of M-theory admit generalized Heisenberg (2-step nilpotent) isometry groups. These metrics may be obtained from known cohomogeneity one metrics of special holonomy by taking a "Heisenberg limit", based on an In\"on\"u-Wigner contraction of the isometry group. Associated with each such metric is an Einstein metric with negative cosmological constant on a solvable group manifold. We discuss the relevance of our metrics to the resolution of singularities in domain-wall spacetimes and some applications to holography. The extremely simple forms of the explicit metrics suggest that they will be useful for many other applications. We also give new but incomplete inhomogeneous metrics of holonomy SU(3), $G_2$ and Spin(7), which are $T_1$, $T_2$ and $T_3$ bundles respectively over hyper-K\"ahler four-manifolds.
Special Hermitian metrics and Lie groups  [PDF]
Nicola Enrietti,Anna Fino
Mathematics , 2011,
Abstract: A Hermitian metric on a complex manifold is called strong K\"ahler with torsion (SKT) if its fundamental 2-form $\omega$ is $\partial \bar \partial$-closed. We review some properties of strong KT metrics also in relation with symplectic forms taming complex structures. Starting from a $2n$-dimensional SKT Lie algebra $\mathfrak g$ {and using} a Hermitian flat connection on $\mathfrak g$ we construct a $4n$-dimensional SKT Lie algebra. We apply this method to some 4-dimensional SKT Lie algebras. Moreover, we classify symplectic forms taming complex structures on 4-dimensional Lie algebras.
Isolated singularities of affine special K?hler metrics in two dimensions  [PDF]
Andriy Haydys
Mathematics , 2015, DOI: 10.1007/s00220-015-2441-6
Abstract: We prove that there are just two types of isolated singularities of special K\"ahler metrics in real dimension two provided the associated holomorphic cubic form does not have essential singularities. We also construct examples of such metrics.
Termhood-based Comparability Metrics of Comparable Corpus in Special Domain  [PDF]
Sa Liu,Chengzhi Zhang
Computer Science , 2013, DOI: 10.1007/978-3-642-36337-5_15
Abstract: Cross-Language Information Retrieval (CLIR) and machine translation (MT) resources, such as dictionaries and parallel corpora, are scarce and hard to come by for special domains. Besides, these resources are just limited to a few languages, such as English, French, and Spanish and so on. So, obtaining comparable corpora automatically for such domains could be an answer to this problem effectively. Comparable corpora, that the subcorpora are not translations of each other, can be easily obtained from web. Therefore, building and using comparable corpora is often a more feasible option in multilingual information processing. Comparability metrics is one of key issues in the field of building and using comparable corpus. Currently, there is no widely accepted definition or metrics method of corpus comparability. In fact, Different definitions or metrics methods of comparability might be given to suit various tasks about natural language processing. A new comparability, namely, termhood-based metrics, oriented to the task of bilingual terminology extraction, is proposed in this paper. In this method, words are ranked by termhood not frequency, and then the cosine similarities, calculated based on the ranking lists of word termhood, is used as comparability. Experiments results show that termhood-based metrics performs better than traditional frequency-based metrics.
Explicit Quaternionic Contact Structures and Metrics with Special Holonomy  [PDF]
Luis C. de Andres,Marisa Fernandez,Stefan Ivanov,Jose A. Santisteban,Luis Ugarte,Dimiter Vassilev
Mathematics , 2009,
Abstract: We construct explicit left invariant quaternionic contact structures on Lie groups with zero and non-zero torsion, and with non-vanishing quaternionic contact conformal curvature tensor, thus showing the existence of quaternionic contact manifolds not locally quaternionic contact conformal to the quaternionic sphere. We present a left invariant quaternionic contact structure on a seven dimensional non-nilpotent Lie group, and show that this structure is locally quaternionic contact conformal to the flat quaternionic contact structure on the quaternionic Heisenberg group. On the product of a seven dimensional Lie group, equipped with a quaternionic contact structure, with the real line we determine explicit complete quaternionic Kaehhler metrics and $Spin(7)$-holonomy metrics which seem to be new. We give explicit complete non-compact eight dimensional almost quaternion hermitian manifolds with closed fundamental four form which are not quaternionic K\"ahler.
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