Sharp Hausdorff Bounds for the Interior Singular Set of Convex $k$-Hessian Solutions

Abstract

Let $2\le k\le n$, let $\Omega \subset {\mathbb{R}}^n$ be open and convex, and let $u$ be a convex viscosity solution of ${\sigma }_k(D^{2}u)=1$ in $\Omega$. We prove that the set on which $u$ fails to be locally $C^2$ has vanishing $(n-1)$-dimensional Hausdorff measure. In the intermediate range $3\le k<n$, this gives a codimension-one refinement of the known almost-everywhere partial regularity, and the exponent is sharp. More generally, for a convex viscosity subsolution of ${\sigma }_k(D^{2}u)\ge \lambda >0$, we obtain Hausdorff bounds for strata defined by the affine dimension of all supporting contact sets. The proof combines a support-dependent Chou--Wang barrier argument, an estimate for the product of the smallest $k$ semiaxes of a John ellipsoid, and Mooney's convex section-covering theorem. As a direct analytical consequence, the full distributional Hessian is absolutely continuous and $u\in W_{\mathrm{loc}}^{2,1}(\Omega )$, yielding a $k$-Hessian counterpart of the $W^{2,1}$ regularity known for singular Monge--Ampère solutions. In a logically separate structural part, we characterize the distinguished number of flat directions, $n-k+1$, by an asymptotic infimum mean-value formula over affine sections, and explain how this mean-value heuristic leads to the supporting-contact geometry used in the proof.

34 pages, 2 figures. Comments, questions, and corrections are welcome. A bilingual expository note with interactive figures is available at https://hxypqr.github.io/post.html?id=113