On a special type of generalized Berwald manifolds: semi-symmetric linear connections preserving the Finslerian length of tangent vectors

Abstract

A generalized Berwald manifold is a Finsler manifold admitting a linear connection on the base manifold such that parallel transports preserve the Finslerian length of tangent vectors (compatibility condition). If the linear connection preserving the Finslerian length of tangent vectors has zero torsion then we have a classical Berwald manifold. Another important type of generalized Berwald manifolds admits semi-symmetric compatible linear connections. This means that the torsion tensor is decomposable in a special way. Among others the so-called Wagner manifolds belong to this special class of spaces. In the sense of Hashiguchi and Ychijyo’s classical results, Wagner manifolds play an important role in the conformal Finsler geometry as conformally Berwald Finsler manifolds. One of the main problems is the intrinsic characterization of compatible linear connections on a Finsler manifold. We are also interested in the inverse problem of compatible linear connections on Finsler manifolds. Let a metric linear connection $$\nabla $$ on a Riemannian manifold be given. After formulating a necessary and sufficient condition for $$\nabla $$ to be metrizable by a non-Riemannian metric function we present a geometric construction of the (non-Riemannian) indicatrix hypersurfaces in terms of generalized conics—in case of a Riemannian manifold the indicatrices are conics (quadratic hypersurfaces) in the classical sense. New perspectives of the theory of generalized Berwald manifolds have been supported by the solution of Matsumoto’s problem of conformally equivalent Berwald manifolds in 2005: the scale function between two non-Riemannian (classical) Berwald manifolds must be constant. The proof is based on metrics and differential forms given by averaging. They also play the central role in the intrinsic characterization of semi-symmetric compatible linear connections in general. Using average processes is a new and important trend in Finsler geometry. The so-called associated Riemannian metric is introduced by choosing the Riemann–Finsler metric to be averaged. Another important type of associated objects is the so-called associated Randers metric. The one-form perturbation of the associated Riemannian metric is given by the integration of the contracted-normalized Riemann–Finsler metric on the indicatrix hypersurface point by point. Some applications are also presented in case of Funk metrics. Since the associated objects inherit the compatibility properties, the generalized Berwald manifold theory for spaces of special metrics is of special interest. In what follows we present some recent results of the theory of generalized Berwald manifolds. Especially we focus on the case of semi-symmetric compatible linear connections. We also discuss the case of Randers metrics. Asanov’s Finsleroid-Finsler metrics will be characterized as the solutions of a conformal rigidity problem and we prove that a Finsleroid-Finsler manifold is a Landsberg manifold (Unicorn) if and only if it is a generalized Berwald manifold with a semi-symmetric compatible linear connection.