Finsler manifold

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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, that enables one to define the length of any smooth curve γ : [a, b] → M as

Contents

Finsler manifolds are more general than Riemannian manifolds since the tangent norms need not be induced by inner products.

Every Finsler manifold becomes an intrinsic quasimetric space when the distance between two points is defined as the infimum length of the curves that join them.

ÉlieCartan  ( 1933 ) named Finsler manifolds after Paul Finsler, who studied this geometry in his dissertation ( Finsler 1918 ).

Definition

A Finsler manifold is a differentiable manifold M together with a Finsler metric, which is a continuous nonnegative function F: TM → [0, +∞) defined on the tangent bundle so that for each point x of M,

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:

The subadditivity axiom may then be replaced by the following strong convexity condition:

Here the Hessian of F2 at v is the symmetric bilinear form

also known as the fundamental tensor of F at v. Strong convexity of implies the subadditivity with a strict inequality if uF(u)vF(v). If F is strongly convex, then it is a Minkowski norm on each tangent space.

A Finsler metric is reversible if, in addition,

A reversible Finsler metric defines a norm (in the usual sense) on each tangent space.

Examples

Randers manifolds

Let be a Riemannian manifold and b a differential one-form on M with

where is the inverse matrix of and the Einstein notation is used. Then

defines a Randers metric on M and is a Randers manifold, a special case of a non-reversible Finsler manifold. [1]

Smooth quasimetric spaces

Let (M, d) be a quasimetric so that M is also a differentiable manifold and d is compatible with the differential structure of M in the following sense:

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

and in fact any Finsler function F: TM  [0, ∞) defines an intrinsic quasimetric dL on M by this formula.

Geodesics

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

of the Jacobi equation for a general spray structure (M, H) in terms of the Ehresmann curvature and nonlinear covariant derivative.

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.

Notes

  1. Randers, G. (1941). "On an Asymmetrical Metric in the Four-Space of General Relativity". Phys. Rev. 59 (2): 195–199. doi:10.1103/PhysRev.59.195. hdl: 10338.dmlcz/134230 .

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