Narrow Results

In this paper, we study a non-collapsed Gromov–Hausdorff limit of a sequence of compact Heisenberg manifolds with sub-Riemannian metrics. In the case of strictly sub-Riemannian case, we show that if a sequence has an upper bound of the diameter and a lower bound of Popp’s measure, then it has a convergent subsequence in the Gromov–Hausdorff topology, and the limit is isometric to a compact Heisenberg manifold of the same dimension. The same conclusion is also shown for Riemannian case with the additional assumption on Ricci curvature lower bounds.

The field equations of gravity coupled to electromagnetism and equations of motion of a charged particle are part of the Kaluza–Klein theory where general relativity is extended to five dimensions. These equations can also be obtained if nonholonomic constrains are imposed on the 5-vector of particle’s velocity. Hence, further development of the general relativity theory can be sub-Riemannian (or sub-Lorentzian) geometry. Gauge transformations become a special case of coordinate transformations in both the Kaluza–Klein theory and the nonholonomic model. Sub-Riemannian geodesics are proved to be equations of motion of a charged particle. The Dirac operator can be extended for a 5-dimensional manifold as a first-order differential operator. Since the base manifold in physics contains the electromagnetic gauge group $U(1)$, the eigenvalues of the charge operator are always an integer multiplied by the fundamental electric charge.

We prove that H-type Carnot groups of rank $k$ and dimension $n$ satisfy the $\mathrm{\text{MCP}}(K,N)$ if and only if $K\le 0$ and $N\ge k+3(n-k)$. The latter integer coincides with the geodesic dimension of the Carnot group. The same result holds true for the larger class of generalized H-type Carnot groups introduced in this paper, and for which we compute explicitly the optimal synthesis. This constitutes the largest class of Carnot groups for which the curvature exponent coincides with the geodesic dimension. We stress that generalized H-type Carnot groups have step 2, include all corank 1 groups and, in general, admit abnormal minimizing curves. As a corollary, we prove the absolute continuity of the Wasserstein geodesics for the quadratic cost on all generalized H-type Carnot groups.

In ${ℝ}^3$ we consider the vector fields

We prove a Steiner formula for regular surfaces with no characteristic points in 3D contact sub-Riemannian manifolds endowed with an arbitrary smooth volume. The formula we obtain, which is equivalent to a half-tube formula, is of local nature. It can thus be applied to any surface in a region not containing characteristic points. We provide a geometrical interpretation of the coefficients appearing in the expansion, and compute them on some relevant examples in three-dimensional sub-Riemannian model spaces. These results generalize those obtained in [Z. M. Balogh, F. Ferrari, B. Franchi, E. Vecchi and K. Wildrick, Steiner’s formula in the Heisenberg group, Nonlinear Anal. 126 (2015) 201–217; M. Ritoré, Tubular neighborhoods in the sub-Riemannian Heisenberg groups, Adv. Calc. Var. 14(1) (2021) 1–36] for the Heisenberg group.

We study the geometry of a sub-Riemannian manifold (M, HM, VM, g), where HM and VM are the horizontal and vertical distribution respectively, and g is a Riemannian extension of the Riemannian metric on HM. First, without the assumption that HM and VM are orthogonal, we construct a sub- Riemannian connection ▽ on HM and prove some Bianchi identities for ▽. Then, we introduce the horizontal sectional curvature, prove a Schur theorem for sub-Riemannian geometry and find a class of sub-Riemannian manifolds of constant horizontal curvature. Finally, we define the horizontal Ricci tensor and scalar curvature, and some sub-Riemannian differential operators (gradient, divergence, Laplacian), extending some results from geometry to the sub-Riemannian setting.

We consider nilpotent Lie groups for which the derived subgroup is abelian. We equip them with sub-Riemannian metrics and we study the normal Hamiltonian flow on the cotangent bundle. We show a correspondence between normal trajectories and polynomial Hamiltonians in some Euclidean space. We use the aforementioned correspondence to give a criterion for the integrability of the normal Hamiltonian flow. As an immediate consequence, we show that in Engel-type groups the flow of the normal Hamiltonian is integrable. For Carnot groups that are semidirect products of two abelian groups, we give a set of conditions that normal trajectories must fulfill to be globally length-minimizing. Our results are based on a symplectic reduction procedure.

The well known Foucault pendulum is studied within the formalism of sub-Riemannian geometry on a step-2 nilpotent Lie group. For small oscillations, trajectories are explicitly calculated, they turn out to be hypotrochöids obtained by rolling without slipping a circle onto another circle.

We introduce a notion of convexity in the geometry of vector fields and we prove a PDE-characterization for such notion and related properties.