A Decomposition Lemma in Convex Integration via Classical Algebraic Geometry

Abstract

In this paper, we prove a decomposition lemma for symmetric matrix fields on bounded domains: $D+\mathrm{Sym}\nabla\Phi=\sum_i a_i^2\xi_i\otimes\xi_i$ with uniform control on $\Phi$ and $a_i^2$, using fewer than the usual $n(n+1)/2$ rank-one symmetric terms. Except possibly in dimensions $n=8,16$, the decomposition is shown to be optimal through algebraic arguments. This reduces the number of steps in convex integration for a nonlinear PDE system, improving H\"older regularity of flexible solutions in dimension $n\ge3$. This PDE is a partial linearization of the codimension-one local isometric embedding equation in the Nash--Kuiper theorem, and also yields improved regularity for very weak solutions of related 2D Monge--Amp\'ere and $2$-Hessian systems. The improved H\"older exponent is any $\alpha<(n^2+1)^{-1}$ for $n=2,4,8,16$ and any $\alpha<(n^2+n-2\rho(n/2)-1)^{-1}$ otherwise, where $\rho$ is the Radon--Hurwitz number, related to Bott periodicity. The proof involves novel applications of algebraic geometry and topology that yield the optimality of decomposition, including Adams' theorem on vector fields on spheres, intersections of projective varieties, and projective duality, combined with an elliptic method that avoids loss of differentiability.