This paper is concerned with developing accurate and efficient nonstandard discontinuous Galerkin methods for fully nonlinear second order elliptic and parabolic partial differential equations (PDEs) in the case of one spatial dimension. The primary goal of the paper to develop a general framework for constructing high order local discontinuous Galerkin (LDG) methods for approximating viscosity solutions of these fully nonlinear PDEs which are merely continuous functions by definition. In order to capture discontinuities of the first order derivative ux\documentclass[12pt]{minimal}
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\begin{document}$$q^+$$\end{document} are introduced to approximate one-sided derivatives of u\documentclass[12pt]{minimal}
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\begin{document}$$u$$\end{document}. Similarly, to capture the discontinuities of the second order derivative uxx\documentclass[12pt]{minimal}
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\begin{document}$$p^{+ +}$$\end{document} are used to approximate one-sided derivatives of q-\documentclass[12pt]{minimal}
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\begin{document}$$q^+$$\end{document}. The proposed LDG framework, which is based on a nonstandard mixed formulation of the underlying PDE, embeds a given fully nonlinear problem into a mostly linear system of equations where the given nonlinear differential operator must be replaced by a numerical operator which allows multiple value inputs of the first and second order derivatives ux\documentclass[12pt]{minimal}
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\begin{document}$$u_{xx}$$\end{document}. An easy to verify set of criteria for constructing “good” numerical operators is also proposed. It consists of consistency and generalized monotonicity. To ensure such a generalized monotonicity property, the crux of the construction is to introduce the numerical moment in the numerical operator, which plays a critical role in the proposed LDG framework. The generalized monotonicity gives the LDG methods the ability to select the viscosity solution among all possible solutions. The proposed framework extends a companion finite difference framework developed by Feng and Lewis (J Comp Appl Math 254:81–98, 2013) and allows for the approximation of fully nonlinear PDEs using high order polynomials and non-uniform meshes. Numerical experiments are also presented to demonstrate the accuracy, efficiency and utility of the proposed LDG methods.
