Area-minimizing properties of Pansu spheres in the sub-riemannian $3$-sphere

TL;DR

This paper proves that certain spherical surfaces in the sub-Riemannian 3-sphere minimize area and solve the isoperimetric problem, revealing their optimality and uniqueness properties within this geometric setting.

Contribution

It establishes the area-minimizing property of Pansu spheres and characterizes solutions to the isoperimetric problem in the sub-Riemannian 3-sphere, using calibration techniques.

Findings

Half-spheres minimize sub-Riemannian area with fixed boundary.

Only vertical translations of half-spheres solve the Plateau problem.

Spheres with positive mean curvature uniquely solve the isoperimetric problem.

Abstract

We consider the sub-Riemannian $3$-sphere $(\mathbb{S}^3,g_h)$ obtained by restriction of the Riemannian metric of constant curvature $1$ to the planar distribution orthogonal to the vertical Hopf vector field. It is known that $(\mathbb{S}^3,g_h)$ contains a family of spherical surfaces $\{\mathcal{S}_\lambda\}_{\lambda\geq 0}$ with constant mean curvature $\lambda$. In this work we first prove that the two closed half-spheres of $\mathcal{S}_0$ with boundary $C_0=\{0\}\times\mathbb{S}^1$ minimize the sub-Riemannian area among compact $C^1$ surfaces with the same boundary. We also see that the only $C^2$ solutions to this Plateau problem are vertical translations of such half-spheres. Second, we establish that the closed $3$-ball enclosed by a sphere $\mathcal{S}_\lambda$ with $\lambda>0$ uniquely solves the isoperimetric problem in $(\mathbb{S}^3,g_h)$ for $C^1$ sets inside a vertical solid tube and containing a horizontal section of the tube. The proofs mainly rely on calibration arguments.