Sharp Hessian integrability for fully nonlinear elliptic supersolutions in low dimensions

We settle the planar sharp Hessian-integrability conjecture of Armstrong, Silvestre, and Smart [\emph{Comm. Pure Appl. Math.} \textbf{65} (2012), 1169--1184] for viscosity supersolutions of fully nonlinear uniformly elliptic equations. If $κ=Λ/λ$, then the optimal exponent in the $W^{2,ε}$ regularit