# 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 ${\displaystyle M}$  we mean a subbundle of the tangent bundle of ${\displaystyle M}$ .

Given a distribution ${\displaystyle H(M)\subset T(M)}$  a vector field in ${\displaystyle H(M)}$  is called horizontal. A curve ${\displaystyle \gamma }$  on ${\displaystyle M}$  is called horizontal if ${\displaystyle {\dot {\gamma }}(t)\in H_{\gamma (t)}(M)}$  for any ${\displaystyle t}$ .

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

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

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

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

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

## Examples

A position of a car on the plane is determined by three parameters: two coordinates ${\displaystyle x}$  and ${\displaystyle y}$  for the location and an angle ${\displaystyle \alpha }$  which describes the orientation of the car. Therefore, the position of the car can be described by a point in a manifold

${\displaystyle \mathbb {R} ^{2}\times S^{1}.}$

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

${\displaystyle \mathbb {R} ^{2}\times S^{1}.}$

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

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

spans the entire algebra. The horizontal distribution ${\displaystyle H}$  spanned by left shifts of ${\displaystyle \alpha }$  and ${\displaystyle \beta }$  is completely non-integrable. Then choosing any smooth positive quadratic form on ${\displaystyle H}$  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.