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# Communications in Analysis and Geometry

## Volume 25 (2017)

### Number 2

### Homotopy properties of horizontal path spaces and a theorem of Serre in subriemannian geometry

Pages: 269 – 301

DOI: http://dx.doi.org/10.4310/CAG.2017.v25.n2.a1

#### Authors

#### Abstract

We discuss homotopy properties of endpoint maps for *horizontal* path spaces, i.e. spaces of curves on a manifold $M$ whose velocities are constrained to a subbundle $\Delta \subset TM$ in a nonholonomic way. We prove that for every $1 \leq p \lt \infty$ these maps are Hurewicz fibrations with respect to the $W^{1,p}$ topology on the space of trajectories.

We prove that the space of horizontal curves joining any two points (with the induced $W^{1,p}$ topology) has the homotopy type of a CW-complex and its inclusion into the standard path space (i.e. with no nonholonomic constraints) is a homotopy equivalence. We derive topological implications on the local structure of these spaces (even near abnormal curves, whose possible existence is not excluded from our constructions).

We consider indeed the more general class of *affine control* systems, for which the above theorems hold for all $1 \leq p \lt p_c$ (here $p_c \gt 1$ depends only the *step* of the system).

We study critical points of geometric costs for these affine control systems, proving that if the base manifold is compact and there are no abnormal trajectories, then the number of their critical points is infinite (we use Lusternik–Schnirelmann category combined with the Hurewicz property). In the special case where the control system is *subriemannian* this result can be read as the corresponding version of Serre’s theorem.

Paper received on 26 February 2015.