Free Boundary Stable Minimal Hypersurfaces in Positively Curved 4-Manifolds
Yujie Wu
TL;DR
This work extends rigidity results for stable minimal hypersurfaces to the free-boundary setting in positively curved 4-manifolds by adapting the $\mu$-bubble method under $R\ge 2$ and $\mathrm{Ric}_2\ge 0$, with weakly convex boundary and weakly bounded geometry. The authors develop end-parabolic theory for mixed Dirichlet-Neumann problems, establish a one-end constraint on nonparabolic ends, and use $\mu$-bubbles to obtain diameter and volume-growth controls that force rigidity. Consequently, any complete stable two-sided free boundary minimal immersion in such ambient manifolds is totally geodesic with $\mathrm{Ric}(\eta,\eta)=0$ along $M$ and $A(\eta,\eta)=0$ along $\partial M$, leading to nonexistence in compact positively curved settings with weakly convex boundary; a counterexample shows the convexity assumption is essential. The paper also provides a versatile toolkit for free boundary minimal geometry in 4-manifolds, highlighting the role of boundary convexity and intermediate-curvature conditions in rigidity phenomena.
Abstract
We show that the combination of nonnegative 2-intermediate Ricci Curvature and strict positivity of scalar curvature forces rigidity of two-sided free boundary stable minimal hypersurface in a 4-manifold with bounded geometry and weakly convex boundary. This extends the method of Chodosh-Li-Stryker to free boundary minimal hypersurfaces in ambient manifolds with boundary.
