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This paper introduces a description of a doubly warped product manifold, taking into account certain conditions related to the projective curvature tensor. We demonstrate that the factor manifolds of a projectively flat (symmetric) doubly warped product manifold possess constant sectional curvature. In the flatness scenario, a doubly warped product manifold reduces to a singly warped product manifold. We establish that the factor manifolds of a doubly warped product manifold with harmonic projective curvature tensor are Einstein manifolds and exhibit harmonic projective curvature tensor. In Sec. VI, we provide evidence that a projectively flat (symmetric) generalized Robertson–Walker space-time is both a perfect fluid and static. [ABSTRACT FROM AUTHOR]

The section 1 of this paper deals with the definition for a pseudo symmetric spacetime. In section 2, it is proved that a (PZS)4 spacetime satisfying Codazzi type of Z-tensor does not exist. In which condition a (PZS)4 spacetime can be a perfect fluid has been found as a necessary and sufficient condition. After that, special properties are obtained if the (PZS)4 spacetime has harmonic conformal curvature tensor. [ABSTRACT FROM AUTHOR]

In this article, we initiate the study of conformal vector fields in paracontact geometry. First, we establish that if the Reeb vector field ζ is a conformal vector field on N(k)-paracontact metric manifold, then the manifold becomes a para-Sasakian manifold. Next, we show that if the conformal vector field V is pointwise collinear with the Reeb vector field ζ, then the manifold recovers a para-Sasakian manifold as well as V is a constant multiple of ζ. Furthermore, we prove that if a 3-dimensional N(k)-paracontact metric manifold admits a Killing vector field V, then either it is a space of constant sectional curvature k or, V is an infinitesimal strict paracontact transformation. Besides these, we also investigate conformal vector fields on para-Kenmotsu manifolds. [ABSTRACT FROM AUTHOR]