A Bernstein-type theorem for Riemannian manifolds with a Killing field

The classical Bernstein theorem asserts that any complete minimal surface in Euclidean space that can be written as the graph of a function on must be a plane. In this paper, we extend Bernstein’s result to complete minimal surfaces in (may be non-complete) ambient spaces of non-negative Ricci curvature carrying a Killing field. This is done under the assumption that the sign of the angle function between a global Gauss map and the Killing field remains unchanged along the surface. In fact, our main result only requires the presence of a homothetic Killing field. L.J. Alías was partially supported by MEC/FEDER project MTM2004-04934-C04-02, F. Séneca project 00625/PI/04, and F. Séneca grant 01798/EE/05, Spain     

Killing spinors on K?hler manifolds

In the paper Kählerian Killing spinors are defined and their basic properties are investigated. Each Kähler manifold that admits a Kählerian Killing spinor is Einstein of odd complex dimension. Kählerian Killing spinors are a special kind of Kählerian twistor spinors. Real Kählerian Killing spinors appear for example, on closed Kähler manifolds with the smallest possible first eigenvalue of the Dirac operator. For the complex projective spaces P ^2l–1 and the complex hyperbolic spaces H ^2l–1 withl>1 the dimension of the space of Kählerian Killing spinors is equal to ( ). It is shown that in complex dimension 3 the complex hyperbolic space H ³ is the only simple connected complete spin Kähler manifold admitting an imaginary Kählerian Killing spinor.     

Smooth skew morphisms of dicyclic groups

Journal of Algebraic Combinatorics - A skew morphism of a finite group A is a permutation $$\varphi $$ on A fixing the identity element of A, and for which there exists an integer-valued function...     

$$L^\mathrm{2}$$ L 2 -transverse conformal Killing forms on complete foliated manifolds

Mathematische Zeitschrift - In this article, we study the $$L^2$$ -transverse conformal Killing forms on complete foliated Riemannian manifolds and prove some vanishing theorems. Also, we study the...     

Skew Killing spinors

We study the existence of a skew Killing spinor on 2- and 3-dimensional Riemannian spin manifolds. We establish the integrability conditions and prove that these spinor fields correspond to twistor spinors in the two dimensional case while, up to a conformal change of the metric, they correspond to parallel spinors in the three dimensional case.     

Simple modules and their essential extensions for skew polynomial rings

Mathematische Zeitschrift - Let R be a commutative Noetherian ring and $$\alpha $$ an automorphism of R. This paper addresses the question: when does the skew polynomial ring $$S = R[\theta ;...     

Skew Boolean algebras derived from generalized Boolean algebras

Every skew Boolean algebra S has a maximal generalized Boolean algebra image given by S/ where is the Green’s relation defined initially on semigroups. In this paper we study skew Boolean algebras constructed from generalized Boolean algebras B by a twisted product construction for which . In particular we study the congruence lattice of with an eye to viewing as a minimal skew Boolean cover of B. This construction is the object part of a functor from the category GB of generalized Boolean algebras to the category LSB of left-handed skew Boolean algebras. Thus we also look at its left adjoint functor . This paper was written while the second author was a Visiting Professor in the Department of Education at the University of Cagliari. The facilities and assistance provided by the University and by the Department are gratefully acknowledged.     

Central Extensions of *-ordered Skew Fields

A *-ordering of a skew field D induces an ordering of the field K of its central symmetric elements. Let F be an ordered field extension of K. We prove that the central extension of D by F exists and admits a *-ordering extending the given *-ordering of D and ordering of F. As a corollary, we show that every *-ordered skew field can be extended to a *-ordered skew field containing in its center.     

The monoid of $$2 \times 2$$2×2 triangular boolean matrices under skew transposition is non-finitely based

Semigroup Forum - Let $$\mathscr {T\!B}_n$$ be the involution semigroup of all upper triangular boolean $$n\times n$$ matrices under the ordinary matrix multiplication and the skew transposition....     

AUTOMORPHISMS OF SL (2, K) OVER SKEW FIELDS

In this paper, the author proves the following resu: It Let K be a skew field and A be an automorphism of SL(2, K). Then there exists A∈GL(2, K), an automorphism σ or an anti-automorphism τ of K, such that A is of theform AX=AX~σA~(-1) for all X∈SL(2, K)or AX=A(X~τ~2)~(-1)A~(-1) for all X∈SL(2, K),where X~σ, X~τ are the matrices obtained by applying σ, τ on X respee tively and X' is thetranspose of X.     

The Axiom of \Phi -Holomorphic Planes for Normal Killing Type Manifolds

We study the properties of the -holomorphic sectional curvature of normal Killing type manifolds satisfying the axiom of -holomorphic planes.     

Rational operators of the space of formal series

The main result of this paper is the following theorem: the group ring of the universal covering of the group SL(2, ℝ) is embeddable in a skew field with valuation in the sense of Mathiak and the valuation ring is an exceptional chain order in the skew field , i.e., there exists a prime ideal that is not completely prime. In this ring, every divisorial right fractional ideal is principal, and the linearly ordered set of all divisorial fractional right ideals is isomorphic to the real line. This theorem is a consequence of the fact that the universal covering group satisfies sufficient conditions for the embeddability of the group ring of a left ordered group in a skew field. __________ Translated from Fundamentalnaya i Prikladnaya Matematika, Vol. 12, No. 3, pp. 9–53, 2006.     

Canonical connections on paracontact manifolds

The canonical paracontact connection is defined and it is shown that its torsion is the obstruction the paracontact manifold to be paraSasakian. A -homothetic transformation is determined as a special gauge transformation. The η-Einstein manifold are defined, it is proved that their scalar curvature is a constant, and it is shown that in the paraSasakian case these spaces can be obtained from Einstein paraSasakian manifolds with -homothetic transformations. It is shown that an almost paracontact structure admits a connection with totally skew-symmetric torsion if and only if the Nijenhuis tensor of the paracontact structure is skew-symmetric and the defining vector field is Killing.      

The Dirichlet problem for constant mean curvature surfaces in Heisenberg space

We study constant mean curvature graphs in the Riemannian three- dimensional Heisenberg spaces . Each such is the total space of a Riemannian submersion onto the Euclidean plane with geodesic fibers the orbits of a Killing field. We prove the existence and uniqueness of CMC graphs in with respect to the Riemannian submersion over certain domains taking on prescribed boundary values. L. J. Alías was partially supported by MEC/FEDER project MTM2004-04934-C04-02 and Fundación Séneca project 00625/PI/04, Spain.     

On a horizontal conformal Killing tensor of degreep in a sasakian space

Summary We deal with a horizontal conformal Killing tensor of degree p in a Sasakian space. After some preparations we prove that a horizontal conformal Killing tensor of odd degree is necessarily Killing. Moreover, we consider horizontal conformal Killing tensor of even degree. The form of the associated tensor is determined completely and a decomposition theorem is proved. Then we give the examples of a conformal Killing tensor of even degree and a special Killing tensor of odd degree with constant l. Entrata in Redazione il 17 luglio 1971.     

On non-negatively curved metrics on open five-dimensional manifolds

Let V ⁿ be an open manifold of non-negative sectional curvature with a soul Σ of co-dimension two. The universal cover of the unit normal bundle N of the soul in such a manifold is isometric to the direct product M ^n-2 × R. In the study of the metric structure of V ⁿ an important role plays the vector field X which belongs to the projection of the vertical planes distribution of the Riemannian submersion on the factor M in this metric splitting . The case n = 4 was considered in [Gromoll, D., Tapp, K.: Geom. Dedicata 99, 127–136 (2003)] where the authors prove that X is a Killing vector field while the manifold V ⁴ is isometric to the quotient of by the flow along the corresponding Killing field. Following an approach of [Gromoll, D., Tapp, K.: Geom. Dedicata 99, 127–136 (2003)] we consider the next case n = 5 and obtain the same result under the assumption that the set of zeros of X is not empty. Under this assumption we prove that both M ³ and Σ³ admit an open-book decomposition with a bending which is a closed geodesic and pages which are totally geodesic two-spheres, the vector field X is Killing, while the whole manifold V ⁵ is isometric to the quotient of by the flow along corresponding Killing field. Supported by the Faculty of Natural Sciences of the Hogskolan i Kalmar (Sweden).     

Killing tensor fields on the 2-torus

A symmetric tensor field on a Riemannian manifold is called a Killing field if the symmetric part of its covariant derivative equals zero. There is a one-to-one correspondence between Killing tensor fields and first integrals of the geodesic flow which depend polynomially on the velocity. Therefore Killing tensor fields relate closely to the problem of integrability of geodesic flows. In particular, the following question is still open: does there exist a Riemannian metric on the 2-torus which admits an irreducible Killing tensor field of rank ≥ 3? We obtain two necessary conditions on a Riemannian metric on the 2-torus for the existence of Killing tensor fields. The first condition is valid for Killing tensor fields of arbitrary rank and relates to closed geodesics. The second condition is obtained for rank 3 Killing tensor fields and pertains to isolines of the Gaussian curvature.     

Gromov's Centralizer Theorem

We study the properties of rigid geometric structures and their relation with those of finite type. The main result proves that for a noncompact simple Lie group G acting analytically on a manifold M preserving a finite volume and either a connection or a geometric structure of finite type there is a nontrivial space of globally defined Killing vector fields on the universal cover that centralize the action of G. Several appplications of this result are provided.     

Skew loops in flat tori

We produce skew loops—loops having no pair of parallel tangent lines—homotopic to any loop in a flat torus or other quotient of R ⁿ . The interesting case here is n = 3. More subtly for any n, we characterize the homotopy classes that will contain a skew loop having a specified loop as tangent indicatrix. A fellowship from the Lady Davis foundation helped support this work.     

σk-Scalar curvature and eigenvalues of the Dirac operator

On a four-dimensional closed spin manifold (M ⁴, g), the eigenvalues of the Dirac operator can be estimated from below by the total σ₂-scalar curvature of M ⁴ as follows: Equality implies that (M ⁴, g) is a round sphere and the corresponding eigenspinors are Killing spinors.Dedicated to Professor Wang Guangyin on the occasion of his 80th birthday.