Second-Order L ∞ Variational Problems and the ∞-Polylaplacian

In this paper we initiate the study of second-order variational problems in L ^∞, seeking to minimise the L ^∞ norm of a function of the hessian.We also derive and study the respective PDE arising as the analogue of the Euler Lagrange equation. Given H ∈ C ¹(R ^n×n _s ), for the functional E ^∞ (u, O) = |H(D ²u)|L ^∞ (O) , u ∈ W ²,∞(Ω), O ⊆ Ω, (1) the associated equation is the fully nonlinear third-order PDE A ² _∞u := (H _X(D ²u)) ^⊗3 : (D ³u) ^⊗2 = 0. (2) Special cases arisewhen H is the Euclidean length of either the full hessian or of the Laplacian, leading to the ∞-polylaplacian and the ∞-bilaplacian respectively. We establish several results for (1) and (2), including existence of minimisers, of absolute minimisers and of "critical point" generalised solutions, proving also variational characterisations and uniqueness. We also construct explicit generalised solutions and perform numerical experiments.