Contact Blow-up Analysis of the Legendrian Mean Curvature Flow and a Smale-type Theorem for Strict Contactomorphisms of Sasakian Spheres

Abstract

We study the Legendrian mean curvature flow in $\eta$-Einstein Sasakian manifolds by parabolic rescalings which preserve the contact structure. In a compact $\eta$-Einstein Sasakian manifold, regular or not, the contact tangent flows at a Type I singularity are, up to vertical translations of their components, self-similar flows of Legendrian self-shrinkers in the Heisenberg group, and they are unions of Legendrian planes. When the Reeb orbits are not closed, as on the irregular Sasaki--Einstein manifolds, the leaf space of the Reeb foliation is not a Kähler manifold or orbifold, and this result does not follow by projecting the flow to a Lagrangian mean curvature flow; its proof combines local transverse estimates with a Gaussian density localised in a flow box of the Reeb foliation. In a regular manifold a Type I singularity can only form at a point of transverse Gaussian density at least $2$. We apply the analysis to strict contactomorphisms $\phi$ of the standard Sasakian sphere ${\mathbb{S}}^{2k+1}$, whose Legendrian graphs lie in the $\eta$-Einstein manifold $({\mathbb{S}}^{2k+1}\times {\mathbb{S}}^{2k+1})/{\mathbb{S}}^1$. The Jacobian of a Legendrian graph obeys a reaction--diffusion equation whose quadratic term is a sum of squares when $k=1$ and is indefinite when $k\ge 2$. For every strict contactomorphism of ${\mathbb{S}}^3$, and for every pinched strict contactomorphism of ${\mathbb{S}}^{2k+1}$, $k\ge 2$, the contact blow-ups of the flow of its Legendrian graph are flat, the flow exists for all time and, up to a rotation along the Reeb flow, converges exponentially to the graph of an element of $U(k+1)$. In particular such a $\phi$ is isotopic to $U(k+1)$ through strict contactomorphisms.