# 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 ${\dot {\gamma }}(t)\in H_{\gamma (t)}(M)$ 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 $A(x),\ [A,B](x),\ [A,[B,C]](x),\ [A,[B,[C,D]]](x),\dotsc \in T_{x}(M)$ where all vector fields $A,B,C,D,\dots$ are horizontal. 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
    $d(x,y)=\inf \int _{0}^{1}{\sqrt {g({\dot {\gamma }}(t),{\dot {\gamma }}(t))}}\,dt,$
where infimum is taken along all horizontal curves $\gamma :[0,1]\to M$ such that $\gamma (0)=x$, $\gamma (1)=y$. Horizontal curves can be taken either Lipschitz continuous, Absolutely continuous or in the Sobolev space $H^{1}([0,1],M)$ 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
    $\{\alpha ,\beta ,[\alpha ,\beta ]\}$
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.