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In mathematics, particularly differential geometry, a Finsler manifold is a differentiable manifold M where a (possibly asymmetric) Minkowski functional F(x,−) is provided on each tangent space TxM, allowing to define the length of any smooth curve γ : [a,b] → M as
- F(v + w) ≤ F(v) + F(w) for every two vectors v,w tangent to M at x (subadditivity).
- F(λv) = λF(v) for all λ ≥ 0 (but not necessarily for λ < 0) (positive homogeneity).
- F(v) > 0 unless v = 0 (positive definiteness).
In other words, F(x,−) is an asymmetric norm on each tangent space TxM. The Finsler metric F is also required to be smooth, more precisely:
- F is smooth on the complement of the zero section of TM.
The subadditivity axiom may then be replaced by the following strong convexity condition:
also known as the fundamental tensor of F at v. Strong convexity of implies the subadditivity with a strict inequality if u⁄F(u) ≠ v⁄F(v). If F is strongly convex, then it is a Minkowski norm on each tangent space.
A Finsler metric is reversible 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.
- Smooth submanifolds (including open subsets) of a normed vector space of finite dimension are Finsler manifolds if the norm of the vector space is 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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- S. Chern: Finsler geometry is just Riemannian geometry without the quadratic restriction, Notices AMS, 43 (1996), pp. 959–63.
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