Viscosity Solutions in Nonlinear Partial Differential Equations

The theory of viscosity solutions has become a fundamental tool in the study of fully nonlinear partial differential equations (PDEs), offering a robust notion of weak solution even when classical differentiability fails. By employing smooth test functions at local extrema, this framework bypasses the need for distributional or Sobolev‐space formulations, ensuring stability under uniform limits and compatibility with maximum principles. Originally devised for first‐order Hamilton–Jacobi equations, the theory now extends to elliptic, parabolic and integro‐differential settings, including degenerate or singular operators. Its strength lies in unifying existence, uniqueness and regularity results under minimal structural hypotheses. Recent advances have broadened the scope to nonlocal operators—such as fractional Laplacians and integral kernels—that model anomalous diffusion, stochastic control and diverse applications in imaging, materials science and finance. Viscosity methods yield comparison principles, Hölder and Lipschitz estimates, and often guide the design of convergent numerical schemes for complex inverse and optimisation problems.

Research from Nature Portfolio

Recent studies have revisited the interplay between stochastic game frameworks and nonlinear PDEs, mapping Tug‐of‐War games to continuum and graph‐based operators, including the infinity‐Laplacian and game p‐Laplacian. This unifying approach provides efficient algorithms for solving inverse problems in imaging and high‐dimensional data analysis, with demonstrated impact in cultural heritage restoration and medical image reconstruction.

Viscosity Solutions in Nonlinear Partial Differential Equations publication trend

The graph below shows the total number of articles in viscosity solutions in nonlinear partial differential equations across all publications each year (not limited to Nature Index journals).

Technical terms

Viscosity solution: A generalised solution concept for nonlinear PDEs defined by comparison with smooth test functions at points of local extremum.

Nonlocal operator: An operator involving integrals over a domain, modelling long-range interactions or anomalous diffusion.

p-Laplacian: A nonlinear differential operator of the form div(|∇u|^{p−2}∇u) that generalises the Laplacian for p≠2.

Comparison principle: A property ensuring that a subsolution never exceeds a supersolution, central to uniqueness proofs.

Perron’s method: A technique for constructing solutions by enveloping subsolutions and supersolutions to establish existence.

References

1. Tug of War games and PDEs on graphs with applications in image and high dimensional data processing. Scientific Reports (2023).
2. Hölder estimates for viscosity solutions of equations of fractional p-Laplace type. Nonlinear Differential Equations and Applications NoDEA (2016).
3. On viscosity and weak solutions for non-homogeneous p-Laplace equations. Advances in Nonlinear Analysis (2017).
4. Viscosity solutions of nonlinear integro-differential equations. Annales de l Institut Henri Poincaré C Analyse Non Linéaire (1996).