In mathematics, particularly differential geometry, a Finsler manifold is a differentiable manifold together with the structure of an intrinsic quasimetric space in which the length of any rectifiable curve γ : [a,b] → M is given by the length functional
where F(x, · ) is a Minkowski norm (or at least an asymmetric norm) on each tangent space TxM. Finsler manifolds non-trivially generalize Riemannian manifolds in the sense that they are not necessarily infinitesimally Euclidean. This means that the (asymmetric) norm on each tangent space is not necessarily induced by an inner product (metric tensor).
- F is smooth on the complement of the zero section of TM.
- F(v) ≥ 0 with equality if and only if v = 0 (positive definiteness).
- F(λv) = λF(v) for all λ ≥ 0 (but not necessarily for λ < 0) (homogeneity).
- F(v + w) ≤ F(v) + F(w) for all w at the same tangent space with v (subadditivity).
In other words, F is an asymmetric norm on each tangent space. Typically one replaces the subadditivity with the following strong convexity condition:
also known as the fundamental tensor of F at v. Strong convexity of F2 implies the subadditivity with a strict inequality if u/F(u) ≠ v/F(v). If F2 is strongly convex, then F is a Minkowski norm on each tangent space.
A Finsler metric is reversible or absolutely homogenous if, in addition,
- F(−v) = F(v) for all tangent vectors v.
A reversible Finsler metric defines a norm (in the usual sense) on each tangent space.
- Normed vector spaces of finite dimension, such as Euclidean spaces, whose norms are smooth outside the origin.
- Riemannian manifolds (but not pseudo-Riemannian manifolds) are special cases of Finsler manifolds.
defines a Randers metric on M and (M,F) is a Randers manifold, a special case of a non-reversible Finsler manifold.
Smooth quasimetric spaces
- Around any point z on M there exists a smooth chart (U, φ) of M and a constant C ≥ 1 such that for every x,y ∈ U
- The function d : M × M →[0,∞] is smooth in some punctured neighborhood of the diagonal.
Then one can define a Finsler function F : TM →[0,∞] by
where γ is any curve in M with γ(0) = x and γ'(0) = v. The Finsler function F obtained in this way restricts to an asymmetric (typically non-Minkowski) norm on each tangent space of M. The induced intrinsic metric dL: M × M → [0, ∞] of the original quasimetric can be recovered from
Due to the homogeneity of F the length
of a differentiable curve γ:[a,b]→M in M is invariant under positively oriented reparametrizations. A constant speed curve γ is a geodesic of a Finsler manifold if its short enough segments γ|[c,d] are length-minimizing in M from γ(c) to γ(d). Equivalently, γ is a geodesic if it is stationary for the energy functional
in the sense that its functional derivative vanishes among differentiable curves γ:[a,b]→M with fixed endpoints γ(a)=x and γ(b)=y.
Canonical spray structure on a Finsler manifold
The Euler–Lagrange equation for the energy functional E[γ] reads in the local coordinates (x1,...,xn,v1,...,vn) of TM as
where k=1,...,n and gij is the coordinate representation of the fundamental tensor, defined as
Assuming the strong convexity of F2(x,v) with respect to v∈TxM, the matrix gij(x,v) is invertible and its inverse is denoted by gij(x,v). Then γ:[a,b]→M is a geodesic of (M,F) if and only if its tangent curve γ':[a,b]→TM \0 is an integral curve of the smooth vector field H on TM \0 locally defined by
where the local spray coefficients Gi are given by
The vector field H on TM/0 satisfies JH = V and [V,H] = H, where J and V are the canonical endomorphism and the canonical vector field on TM \0. Hence, by definition, H is a spray on M. The spray H defines a nonlinear connection on the fibre bundle TM \0 → M through the vertical projection
In analogy with the Riemannian case, there is a version
Uniqueness and minimizing properties of geodesics
By Hopf–Rinow theorem there always exist length minimizing curves (at least in small enough neighborhoods) on (M, F). Length minimizing curves can always be positively reparametrized to be geodesics, and any geodesic must satisfy the Euler–Lagrange equation for E[γ]. Assuming the strong convexity of F2 there exists a unique maximal geodesic γ with γ(0) = x and γ'(0) = v for any (x, v) ∈ TM \ 0 by the uniqueness of integral curves.
If F2 is strongly convex, geodesics γ : [0, b] → M are length-minimizing among nearby curves until the first point γ(s) conjugate to γ(0) along γ, and for t > s there always exist shorter curves from γ(0) to γ(t) near γ, as in the Riemannian case.
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