On the covariant coefficients of geodesic sprays on Finsler manifolds
S. G. Elgendi, A. Soleiman, Nabil L. Youssef
TL;DR
This work introduces the covariant coefficients $H_i$ of the geodesic spray of a Finsler metric $F$ and the spray scalar $H$ when $H_i=\partial H/\partial y^i$. It proves that the existence of $H$ is equivalent to dual flatness and gives the explicit form $H=\frac{1}{12} y^k \partial_k F^2$, with $H$ unique up to a function of position. The paper then analyzes the higher tensors generated from $H_i$, shows that they do not in general form a linear connection but do so for projectively flat metrics, and introduces the notions of $H$-Berwald and $H$-Landsberg spaces, including their interrelations and conditions under which they coincide with classical Berwald/Landsberg spaces. In the special cases of projectively flat and dually flat (notably spherically symmetric) Finsler metrics, the authors solve the $H$-unicorn Landsberg problem by exhibiting metrics for which $H$-Landsberg holds without $H$-Berwaldness. The paper provides explicit examples of both $H$-Berwald and $H$-Landsberg spaces, including a Randers-type metric that is $H$-Landsberg but not $H$-Berwald, and a higher-dimensional $H$-Berwald construction, illustrating the utility of the $H$-framework in Finsler geometry.
Abstract
For a Finsler metric $F$, we introduce the notion of $F$-covariant coefficients $H_i$ of the geodesic spray of $F$ (Def. 3.1). We study some geometric consequences concerning the objects $H_i$. If the $F$-covariant coefficients $H_i$ are written in the form $H_i={\dot{\partial}}_iH$, for some smooth function $H$ on ${\mathcal T\hspace{-1pt}M}$, positively 3-homogeneous in y, then $H$ is called spray scalar or simply $S$-scalar. We prove that if the $S$-scalar exists, then it is of the form $H=\frac{1}{12}\,y^i\partial_iF^2$ and this expression is unique up to a function of position only. We prove also that on a Finsler maifold $(M,F)$, the $S$-scalar $H$ exists if and only if $(M,F)$ is dually flat. Generally, the $n^3$ functions $H^h_{ij}$ resulting from the $F$-covariant coefficients do not form a linear connection. We find out that in the case of projectively flat metrics, the $n^3$ functions $H^h_{ij}$ are coefficients of a linear connection. We introduce two new special Finsler spaces, namely, the $H$-Berwald and the $H$-Landsberg spaces and show that every $H$-Berwald metric is $H$-Landsbergian but the converse is not necessarily true. Also, we study the $F$-covariant coefficients $H_i$ of projectivly flat and dually flat spherically symmetric Finsler metrics and provide a solution of the "$H$-unicorn" Landsberg problem. Finally, we give some examples of $H$-Berwald and $H$-Landsberg metrics and an example of $H$-Landsberg metric which is not $H$-Berwaldian.