# Finsler manifold

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 *T*_{x}*M*. 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).

Template:Harvs named Finsler manifolds after Paul Finsler, who studied this geometry in his dissertation Template:Harv.

## Definition

A **Finsler manifold** is a differentiable manifold *M* together with a **Finsler function** *F* defined on the tangent bundle of *M* so that for all tangent vectors *v*,

*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*(λ*v*) = λ*F*(*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:

- For each tangent vector
*v*, the hessian of*F*^{2}at*v*is positive definite.

Here the hessian of *F*^{2} at *v* is the symmetric bilinear form

also known as the **fundamental tensor** of *F* at *v*. Strong convexity of *F*^{2} implies the subadditivity with a strict inequality if *u*/*F*(*u*) ≠ *v*/*F*(*v*). If *F*^{2} is strongly convex, then *F* 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.

## Examples

- 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.

### Randers manifolds

Let (*M*,*a*) 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 (*M*,*F*) 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:

- 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 *d*_{L}: *M* × *M* → [0, ∞] of the original quasimetric can be recovered from

and in fact any Finsler function *F* : *TM* → [0, ∞) defines an intrinsic quasimetric *d*_{L} 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 (*x*^{1},...,*x*^{n},*v*^{1},...,*v*^{n}) of *TM* as

where *k*=1,...,*n* and *g*_{ij} is the coordinate representation of the fundamental tensor, defined as

Assuming the strong convexity of *F*^{2}(*x,v*) with respect to *v*∈*T _{x}M*, the matrix

*g*

_{ij}(

*x*,

*v*) is invertible and its inverse is denoted by

*g*

^{ij}(

*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 *G*^{i} 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 *F*^{2} 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 *F*^{2} 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

- ↑ {{#invoke:Citation/CS1|citation |CitationClass=journal }}

## References

- {{#invoke:citation/CS1|citation

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- D. Bao, S. S. Chern and Z. Shen,
*An Introduction to Riemann–Finsler Geometry,*Springer-Verlag, 2000. ISBN 0-387-98948-X. - {{#invoke:citation/CS1|citation

|CitationClass=citation }}

- S. Chern:
*Finsler geometry is just Riemannian geometry without the quadratic restriction*, Notices AMS, 43 (1996), pp. 959–63. - {{#invoke:citation/CS1|citation

|CitationClass=citation }} (Reprinted by Birkhäuser (1951))

- H. Rund.
*The Differential Geometry of Finsler Spaces,*Springer-Verlag, 1959. ASIN B0006AWABG. - Z. Shen,
*Lectures on Finsler Geometry,*World Scientific Publishers, 2001. ISBN 981-02-4531-9.

## External links

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