Diffusion and reaction in the quasi-spherical equation: mean curvature deformations for fill-Ins

We study the diffusion and reaction effects of Bartnik’s quasi-spherical equation to deform boundary mean curvature in fill-in problems with scalar curvature bounded below. The diffusion effect yields an explicit $L^p$-to-$L^{\mathrm{\infty }}$ estimate for ${\mathrm{\partial }}_{t}u=u^2\mathrm{\Delta }u$, thereby extending the known upper bound for the minimum boundary mean curvature of fill-ins to a quantitative upper bound for its harmonic mean. For spin fill-ins, this bound is explicit and involves only coarse intrinsic boundary data. By introducing an absorbing reaction term, we also construct a deformation that transforms any nonnegative initial mean curvature into a terminal mean curvature with a uniform positive lower bound. For Gromov’s conjecture on total mean curvature, this reduces the $H\ge 0$ case to Theorem A of Frenck, Hanke, and Hirsch \cite{FHH}, which assumes $H\ge \kappa >0$. This covers the case of non-spin boundaries, complementing their result for spin boundaries (Theorem B) for this conjecture.