Finsler manifold

Template:Short description Template:Redirect Template:Refimprove In mathematics, particularly differential geometry, a Finsler manifold is a differentiable manifold Template:Math where a (possibly asymmetric) Minkowski norm Template:Math is provided on each tangent space Template:Math, that enables one to define the length of any smooth curve Template:Math as

<math>L(\gamma) = \int_a^b F\left(\gamma(t), \dot{\gamma}(t)\right)\,\mathrm{d}t.</math>

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.

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 Template:Math together with a Finsler metric, which is a continuous nonnegative function Template:Math defined on the tangent bundle so that for each point Template:Math of Template:Math,

• Template:Math for every two vectors Template:Math tangent to Template:Math at Template:Math (subadditivity).
• Template:Math for all Template:Math (but not necessarily for Template:Math (positive homogeneity).
• Template:Math unless Template:Math (positive definiteness).

In other words, Template:Math is an asymmetric norm on each tangent space Template:Math. The Finsler metric Template:Math is also required to be smooth, more precisely:

• Template:Math is smooth on the complement of the zero section of Template:Math.

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

• For each tangent vector Template:Math, the Hessian matrix of Template:Math at Template:Math is positive definite.

Here the Hessian of Template:Math at Template:Math is the symmetric bilinear form

<math>\mathbf{g}_v(X, Y) := \frac{1}{2}\left.\frac{\partial^2}{\partial s\partial t}\left[F(v + sX + tY)^2\right]\right|_{s=t=0},</math>

also known as the fundamental tensor of Template:Math at Template:Math. Strong convexity of Template:Math implies the subadditivity with a strict inequality if Template:Math. If Template:Math is strongly convex, then it is a Minkowski norm on each tangent space.

A Finsler metric is reversible if, in addition,

• Template:Math for all tangent vectors v.

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

Examples

• 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.

Randers manifolds

Let <math>(M, a)</math> be a Riemannian manifold and b a differential one-form on M with

<math>\|b\|_a := \sqrt{a^{ij}b_i b_j} < 1,</math>

where <math>\left(a^{ij}\right)</math> is the inverse matrix of <math>(a_{ij})</math> and the Einstein notation is used. Then

<math>F(x, v) := \sqrt{a_{ij}(x)v^i v^j} + b_i(x)v^i</math>

defines a Randers metric on M and <math>(M, F)</math> is a Randers manifold, a special case of a non-reversible Finsler manifold.<ref>Template:Cite journal</ref>

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

  <math> \frac{1}{C}\|\phi(y) - \phi(x)\| \leq d(x, y) \leq C\|\phi(y) - \phi(x)\|.</math>

• 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

<math>F(x, v) := \lim_{t \to 0+} \frac{d(\gamma(0), \gamma(t))}{t},</math>

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 Template:Nowrap of the original quasimetric can be recovered from

<math>d_L(x, y) := \inf\left\{\ \left.\int_0^1 F\left(\gamma(t), \dot\gamma(t)\right) \, dt \ \right| \ \gamma\in C^1([0, 1], M) \ , \ \gamma(0) = x \ , \ \gamma(1) = y \ \right\},</math>

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

<math>L[\gamma] := \int_a^b F\left(\gamma(t), \dot{\gamma}(t)\right)\, dt</math>

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

<math>E[\gamma] := \frac{1}{2}\int_a^b F^2\left(\gamma(t), \dot{\gamma}(t)\right)\, dt</math>

in the sense that its functional derivative vanishes among differentiable curves Template:Nowrap with fixed endpoints Template:Nowrap and Template:Nowrap.

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

<math>

 g_{ik}\Big(\gamma(t), \dot\gamma(t)\Big)\ddot\gamma^i(t) + \left(
              \frac{\partial g_{ik}}{\partial x^j}\Big(\gamma(t), \dot\gamma(t)\Big) -
   \frac{1}{2}\frac{\partial g_{ij}}{\partial x^k}\Big(\gamma(t), \dot\gamma(t)\Big)
 \right) \dot\gamma^i(t)\dot\gamma^j(t) = 0,

</math>

where k = 1, ..., n and g_ij is the coordinate representation of the fundamental tensor, defined as

<math>

g_{ij}(x,v) := g_v\left(\left.\frac{\partial}{\partial x^i}\right|_x, \left.\frac{\partial}{\partial x^j}\right|_x\right). </math>

Assuming the strong convexity of F²(x, v) with respect to v ∈ T_xM, the matrix g_ij(x, v) is invertible and its inverse is denoted by g^ij(x, v). Then Template:Nobreak is a geodesic of (M, F) if and only if its tangent curve Template:Nobreak is an integral curve of the smooth vector field H on TM∖{0} locally defined by

<math>

\left.H\right|_{(x, v)} := \left.v^i\frac{\partial}{\partial x^i}\right|_{(x,v)}\!\! - \left.2G^i(x, v)\frac{\partial}{\partial v^i}\right|_{(x,v)}, </math>

where the local spray coefficients Gⁱ are given by

<math>

G^i(x, v) := \frac{1}{4}g^{ij}(x, v)\left(2\frac{\partial g_{jk}}{\partial x^\ell}(x, v) - \frac{\partial g_{k\ell}}{\partial x^j}(x, v)\right)v^k v^\ell. </math>

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 Template:Nowrap through the vertical projection

<math>v: T(\mathrm{T}M \setminus \{0\}) \to T(\mathrm{T}M \setminus \{0\});\quad v := \frac{1}{2}\big(I + \mathcal{L}_H J\big).</math>

In analogy with the Riemannian case, there is a version

<math>D_{\dot\gamma}D_{\dot\gamma}X(t) + R_{\dot\gamma}\left(\dot\gamma(t), X(t)\right) = 0</math>

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.

See also

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• Global analysis – which uses Hilbert manifolds and other kinds of infinite-dimensional manifolds
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References

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• Template:Citation (Reprinted by Birkhäuser (1951))
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External links

• Template:Springer
• The (New) Finsler Newsletter