** Open problems **

- The Sard conjecture
- Regularity of geodesics
- Small balls

**1. The Sard conjecture **

** 1.1. Statement **

Theorem 1 (Morse 1939 for , Sard 1942)If is of class ,

and this is sharp (Whitney).

Does this theorem generalize to the endpoint map of a smooth control system ?

**Conjecture**. The set of all positions at time of singular paths starting at has measure zero.

**Remark**. There are examples of smooth (even polynomial) functions on which do not satisfy Sard’s theorem. The only infinitesimal version of Sard’s theorem is Smale’s for Fredholm maps.

Conjecture is open for Carot groups (which may be harder).

** 1.2. Positive cases **

Fat distributions have no singular curves but constants.

For rank two distributions in dimension 3, singular curves are contained in the Martinet surface which is known to be countably 2-rectifiable. Conjecturally, the singular values of the endpoint map have Hausdorff dimension . Generically, the horizontal curves on the Martinet surface form a foliation whose singularity are either saddles or foci. At foci, the length of leaves is infinite, so one can ignore them.

** 1.3. The minimizing Sard conjecture **

Let denote the set of points joined to by a minimizing geodesic which is singular. Let denote the set of points joined to by a minimizing geodesic which is singular and not the projection of a normal extremal.

The following partial result turns out to be rather easy.

Proposition 2 (Rifford-Trélat, Agrachev)has empty interior.

Lemma 3Assume that there is a function such that

- is differentiable at ,
- and for all neigboring .

Then there is a unique minimizing geodesic between and , which is the projection of a normal extremal such that .

\proof

Let be the control of some minimizing geodesic. For close to ,

with equality at . By assumption, , with equality at . Therefore minimizes in a neighborhood of , and it is locally unique. So there is such that , is normal, q.e.d.

\proof

of Proposition. Any continuous function has a smooth (even constant) support function at a dense set of points, q.e.d.

**Question**. Can one improve this to full measure ?

**2. Regularity of minimizers **

Projections of normal extremals are smooth.

**Question**. Are abnormal minimizing geodesics of class ?

** 2.1. Partial results **

Theorem 4 (Monti-Leonardi)Consider an equiregular ( all have constant dimension) distribution. Assume that . Then curves with a corner cannot be minimizing.

Theorem 5 (Süssmann)If data are real analytic, singular controls are real analytic on an open dense subset of their interval f definition.

This comes from sub-analytic geometry.

**3. Small balls **

**Question**. Are small spheres homeomorphic to spheres ?

It is true in Carnot groups.

Yuri Baryshnikov claims that the answer is yes in the contact case, but the proof does not seem to be correct.

In the absence of abnormal geodesics, then almost every sphere at is a Lipschitz submanifold.