We discuss the integrability of rank 2 sub-Riemannian structures on
low-dimensional manifolds, and then prove that some structures of that type in
dimension 6, 7 and 8 have a lot of symmetry but no integrals polynomial in
momenta of low degrees, except for those coming from the Killing fields and the
Hamiltonian, thus indicating non-integrability of the corresponding geodesic
flows.Comment: In the second version we restructured the material, improved
non-existence result in dimension 7 (to degree 6 using the modular approach),
and updated references. We also refined the algorithm, and we attach the
corresponding commented Maple file (together with PDF outputs of its work for
several cases) as the supplemen