Semi-concurrent vector fields in Finsler geometry

In the present paper, we give an answer to a question which is closely related to doubly warped product of Finsler metrics: "For each n, is there an n-dimensional Finsler manifold (M, F), admitting a non-constant smooth function f on M such that partial derivative f/partial derivative x(i) partial derivative g(ij)/partial derivative y(k)= 0?". We relate the preceding mentioned condition to different concepts appeared and studied in Finsler geometry. We introduce and investigate the notion of a semi concurrent vector field on a Finsler manifold. We show that some special Finsler manifolds admitting such vector fields turn out to be Riemannian. We prove that Tachibana's characterization of Finsler manifolds admitting a concurrent vector field leads to Riemannian metrics. Various examples for conic Finsler spaces that admit semi-concurrent vector field are presented. (C) 2019 Elsevier B.V. All rights reserved.