Dirichlet duality and the nonlinear Dirichlet problem

We study the Dirichlet problem for fully nonlinear, degenerate elliptic equations of the form F (Hess u ) = 0 on a smoothly bounded domain Ω ⋐ ℝ n . In our approach the equation is replaced by a subset F ⊂ Sym 2 (ℝ n ) of the symmetric n × n matrices with ∂ F ⊆ { F = 0}. We establish the existence and uniqueness of continuous solutions under an explicit geometric “ F ‐convexity” assumption on the boundary ∂Ω. We also study the topological structure of F ‐convex domains and prove a theorem of Andreotti‐Frankel type. Two key ingredients in the analysis are the use of “subaffine functions” and “Dirichlet duality.” Associated to F is a Dirichlet dual set F̃ that gives a dual Dirichlet problem. This pairing is a true duality in that the dual of F̃ is F , and in the analysis the roles of F and F̃ are interchangeable. The duality also clarifies many features of the problem including the appropriate conditions on the boundary. Many interesting examples are covered by these results including: all branches of the homogeneous Monge‐Ampère equation over ℝ, ℂ, and ℍ; equations appearing naturally in calibrated geometry, Lagrangian geometry, and p ‐convex Riemannian geometry; and all branches of the special Lagrangian potential equation.