# 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 $M$ .

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 ${\dot {\gamma }}(t)\in H_{\gamma (t)}(M)$  for any $t$ .

A distribution on $H(M)$  is called completely non-integrable if for any $x\in M$  we have that any tangent vector can be presented as a linear combination of vectors of the following types $A(x),\ [A,B](x),\ [A,[B,C]](x),\ [A,[B,[C,D]]](x),\dotsc \in T_{x}(M)$  where all vector fields $A,B,C,D,\dots$  are horizontal.

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 sub-Riemannian manifold carries the natural intrinsic metric, called the metric of Carnot–Carathéodory, defined as

$d(x,y)=\inf \int _{0}^{1}{\sqrt {g({\dot {\gamma }}(t),{\dot {\gamma }}(t))}}\,dt,$

where infimum is taken along all horizontal curves $\gamma :[0,1]\to M$  such that $\gamma (0)=x$ , $\gamma (1)=y$ .

## 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

$\{\alpha ,\beta ,[\alpha ,\beta ]\}$

spans the entire algebra. The horizontal 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. The existence of geodesics of the corresponding Hamilton–Jacobi equations for the sub-Riemannian Hamiltonian is given by the Chow–Rashevskii theorem.