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Author: Ünal, B

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Now showing 1 - 7 of 7
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    Article
    Citation - WoS: 77
    Citation - Scopus: 85
    Curvature of multiply warped products
    (Elsevier, 2005) Dobarro, F; Ünal, B
    In this paper, we study Ricci-flat and Einstein-Lorentzian multiply warped products. We also consider the case of having constant scalar curvatures for this class of warped products. Finally, after we introduce a new class of space-times called as generalized Kasner space-times, we apply our results to this kind of space-times as well as other relativistic space-times, i.e., Reissner-Nordstrom, Kasner space-times, Banados-Teitelboim-Zanelli and de Sitter black hole solutions. (c) 2004 Elsevier B.V. All rights reserved.
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    Article
    Citation - WoS: 21
    Citation - Scopus: 21
    Special Standard Static Space-Times
    (Pergamon-elsevier Science Ltd, 2004) Dobarro, F; Ünal, B
    Essentially, some conditions for the Riemannian factor and the warping function of a standard static space-time are obtained in order to guarantee that no nontrivial warping function on the Riemannian factor can make the standard static space-time Einstein. (C) 2004 Elsevier Ltd. All rights reserved.
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    Article
    Citation - WoS: 61
    Citation - Scopus: 64
    A curvature condition for a twisted product to be a warped product
    (Springer-verlag, 2001) Fernández-López, M; García-Río, E; Kupeli, DN; Ünal, B
    It is shown that a mixed Ricci-flat twisted product semi-Riemannian manifold can be expressed as a warped product semi-Riemannian manifold. Asa consequence, any Einstein twisted product semi-Riemannian manifold is in fact, a warped product serni-Riemannian manifold.
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    Article
    Citation - WoS: 77
    Citation - Scopus: 74
    Doubly Warped Products
    (Elsevier Science Bv, 2001) Ünal, B
    In this paper we study geodesic completeness of Riemannian doubly warped products and Lorentzian doubly warped products. We give necessary conditions for generalized Robertson-Walker spacetimes with doubly warped product spacial parts to be globally hyperbolic. We also state some results about killing and conformal vector fields of doubly warped products.
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    Article
    Citation - WoS: 31
    Citation - Scopus: 30
    Characterizing Specific Riemannian Manifolds by Differential Equations
    (Springer, 2003) Erkekoglu, F; García-Río, E; Kupeli, DN; Ünal, B
    Some characterizations of certain rank-one symmetric Riemannian manifolds by the existence of nontrivial solutions to certain partial differential equations on Riemannian manifolds are surveyed.
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    Article
    Citation - WoS: 47
    Citation - Scopus: 47
    On a differential equation characterizing Euclidean spheres
    (Academic Press inc Elsevier Science, 2003) García-Río, E; Kupeli, DN; Ünal, B
    A characterization of Euclidean spheres out of complete Riemannian manifolds is made by certain vector fields on complete Riemannian manifolds satisfying a partial differential equation on vector fields. (C) 2003 Elsevier Inc. All rights reserved.
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    Article
    Citation - WoS: 30
    Citation - Scopus: 32
    Geodesic Structure of Standard Static Space-Times
    (Elsevier Science Bv, 2003) Allison, DE; Ünal, B
    The geodesic structure of standard static space-times is studied and conditions are found which imply nonreturning and pseudoconvex geodesic systems. As a consequence, it is shown that if the Riemannian factor manifold F satisfies the nonreturning property and has a pseudoconvex geodesic system and if the warping function f : F --> (0, infinity) is bounded above then the standard static space-time (f)(a, b) x F is geodesically connected. (C) 2002 Elsevier Science B.V. All rights reserved.