Year: 2021
Type: article
Abstract: We investigate the regularity of the free boundary for the Signorini problem in $${\mathbb {R}}^{n+1}$$ R n + 1 . It is known that regular points are $$(n-1)$$ ( n - 1 ) -dimensional and $$C^\infty $$ C ∞ . However, even for $$C^\infty $$ C ∞ obstacles $$\varphi $$ φ , the set of non-regular (or degenerate) points could be very large—e.g. with infinite $${\mathcal {H}}^{n-1}$$ H n - 1 measure. The only two assumptions under which a nice structure result for degenerate points has been established are when $$\varphi $$ φ is analytic, and when $$\Delta \varphi < 0$$ Δ φ < 0 . However, even in these cases, the set of degenerate points is in general $$(n-1)$$ ( n - 1 ) -dimensional—as large as the set of regular points. In this work, we show for the first time that, “usually”, the set of degenerate points is small. Namely, we prove that, given any $$C^\infty $$ C ∞ obstacle, for almost every solution the non-regular part of the free boundary is at most $$(n-2)$$ ( n - 2 ) -dimensional. This is the first result in this direction for the Signorini problem. Furthermore, we prove analogous results for the obstacle problem for the fractional Laplacian $$(-\Delta )^s$$ ( - Δ ) s , and for the parabolic Signorini problem. In the parabolic Signorini problem, our main result establishes that the non-regular part of the free boundary is $$(n-1-\alpha _\circ )$$ ( n - 1 - α ∘ ) -dimensional for almost all times t, for some $$\alpha _\circ > 0$$ α ∘ > 0 . Finally, we construct some new examples of free boundaries with degenerate points.
Authors: Xavier Fernández-Real, Xavier Ros-Oton
Language: Latvian
Subfield: Applied Mathematics
Field: Mathematics
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $2,690
Awards: G149784976, Regularity and Stability in Partial Differential Equations, G1634642163, G513251353, Regularity and singularities in elliptic PDE's: beyond monotonicity formulas, Integro-differential elliptic equations, Regularity issues for the Navier-Stokes equations and for other variational problems, G993971353
OpenAlex ID: W2991666831