A. Agrachev et al., GENERIC SINGULARITIES OF SUB-RIEMANNIAN METRICS ON R(3), Comptes rendus de l'Academie des sciences. Serie 1, Mathematique, 322(4), 1996, pp. 377-384
The purpose of this Note is the local study of Sub-Riemannian metrics
in the most simple case: dimension 3 and contact distribution. Even in
this simple case, there exist fundamental differences with Riemannian
metrics. For instance, conjugate points appear, arbitrarily close to
the origin. We give the full local, generic, classification of singula
rities that can happen for conjugate loci, cut loci and spheres.