sub-Riemannian manifold

In mathematics, a sub-Riemannian manifold is a certain type of generalization of a Riemannian manifold. Roughly speaking, to measure distances in a sub-Riemannian manifold, you are allowed to go only along curves tangent to so-called ''horizontal subspaces''.

Sub-Riemannian manifolds (and so, ''a fortiori'', Riemannian manifolds) carry a natural intrinsic metric called the metric of Carnot–Carathéodory. The Hausdorff dimension of such metric spaces is always an integer and larger than its topological dimension (unless it is actually a Riemannian manifold).

Sub-Riemannian manifolds often occur in the study of constrained systems in classical mechanics, such as the motion of vehicles on a surface, the motion of robot arms, and the orbital dynamics of satellites. Geometric quantities such as the Berry phase may be understood in the language of sub-Riemannian geometry. The Heisenberg group, important to quantum mechanics, carries a natural sub-Riemannian structure.

Definitions

By a ''distribution'' on $M$ we mean a subbundle of the tangent bundle of $M$ (see also distribution).

Given a distribution $H(M)\subset T(M)$ a vector field in $H(M)$ is called ''horizontal''. A curve $\gamma$ on $M$ is called horizontal if $\dot\gamma(t)\in H_(M)$ for any
$t$.

A distribution on $H(M)$ is called ''completely non-integrable'' or ''bracket generating'' if for any $x\in M$ we have that any tangent vector can be presented as a linear combination of Lie brackets of horizontal fields, i.e. vectors of the form A(x),\ ,Bx),\ ,[B,C(x),\ [A,[B,[C,D">,C.html" ;"title=",[B,C">,[B,C(x),\ [A,[B,[C,D">,C">,[B,C<_a>(x),\_[A,[B,[C,D.html" ;"title=",C.html" ;"title=",[B,C">,[B,C(x),\ [A,[B,[C,D">,C.html" ;"title=",[B,C">,[B,C(x),\ [A,[B,[C,Dx),\dotsc\in T_x(M) where all vector fields $A,B,C,D, \dots$ are horizontal. This requirement is also known as Hörmander's condition.

A sub-Riemannian manifold is a triple $(M, H, g)$, where $M$ is a differentiable manifold, $H$ is a completely non-integrable "horizontal" distribution and $g$ is a smooth section of positive-definite quadratic forms on $H$.

Any (connected) sub-Riemannian manifold carries a natural intrinsic metric, called the metric of Carnot–Carathéodory, defined as
:$d(x, y) = \inf\int_0^1 \sqrt \, dt,$
where infimum is taken along all ''horizontal curves'' $\gamma:$, 1\to M such that $\gamma(0)=x$, $\gamma(1)=y$.
Horizontal curves can be taken either Lipschitz continuous, Absolutely continuous or in the Sobolev space H^1( ,1M) producing the same metric in all cases.

The fact that the distance of two points is always finite (i.e. any two points are connected by an horizontal curve) is a consequence of Hörmander's condition known as Chow–Rashevskii theorem.

Examples

A position of a car on the plane is determined by three parameters: two coordinates $x$ and $y$ for the location and an angle $\alpha$ which describes the orientation of the car. Therefore, the position of the car can be described by a point in a manifold
:$\mathbb R^2\times S^1.$

One can ask, what is the minimal distance one should drive to get from one position to another? This defines a Carnot–Carathéodory metric on the manifold
:$\mathbb R^2\times S^1.$

A closely related example of a sub-Riemannian metric can be constructed on a Heisenberg group: Take two elements $\alpha$ and $\beta$ in the corresponding Lie algebra such that
:$\$

spans the entire algebra. The distribution $H$ spanned by left shifts of $\alpha$ and $\beta$ is ''completely non-integrable''. Then choosing any smooth positive quadratic form on $H$ gives a sub-Riemannian metric on the group.

Properties

For every sub-Riemannian manifold, there exists a Hamiltonian, called the sub-Riemannian Hamiltonian, constructed out of the metric for the manifold. Conversely, every such quadratic Hamiltonian induces a sub-Riemannian manifold.

Solutions of the corresponding Hamilton–Jacobi equations for the sub-Riemannian Hamiltonian are called geodesics, and generalize Riemannian geodesics.

See also

* Carnot group, a class of Lie groups that form sub-Riemannian manifolds.
*Distribution
* Hörmander's condition
*Optimal control

References

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{{Riemannian geometry

Metric geometry
Riemannian geometry
Riemannian manifolds