More Like this

1. Computational approaches for extremal geometric eigenvalue problems

   C.-Y. Kao; B. Osting; and E ́. Oudet (January 2023, Handbook of numerical analysis)

   In an extremal eigenvalue problem, one considers a family of eigenvalue problems, each with discrete spectra, and extremizes a chosen eigenvalue over the family. In this chapter, we consider eigenvalue problems defined on Riemannian manifolds and extremize over the metric structure. For example, we consider the problem of maximizing the principal Laplace–Beltrami eigenvalue over a family of closed surfaces of fixed volume. Computational approaches to such extremal geometric eigenvalue problems present new computational challenges and require novel numerical tools, such as the parameterization of conformal classes and the development of accurate and efficient methods to solve eigenvalue problems on domains with nontrivial genus and boundary. We highlight recent progress on computational approaches for extremal geometric eigenvalue problems, including (i) maximizing Laplace–Beltrami eigenvalues on closed surfaces and (ii) maximizing Steklov eigenvalues on surfaces with boundary. 

   more » « less
2. An upper bound for the first nonzero Steklov eigenvalue

   https://doi.org/10.1051/cocv/2024088  

   Li, Xiaolong; Wang, Kui; Wu, Haotian (January 2025, ESAIM: Control, Optimisation and Calculus of Variations)

   Let (Mⁿ,g) be a complete simply connectedn-dimensional Riemannian manifold with curvature bounds Sect_g≤ κ for κ ≤ 0 and Ric_g≥ (n− 1)KgforK≤ 0. We prove that for any bounded domain Ω ⊂Mⁿwith diameterdand Lipschitz boundary, if Ω* is a geodesic ball in the simply connected space form with constant sectional curvature κ enclosing the same volume as Ω, then σ₁(Ω) ≤Cσ₁(Ω*), where σ₁(Ω) and σ₁(Ω*) denote the first nonzero Steklov eigenvalues of Ω and Ω* respectively, andC=C(n, κ,K,d) is an explicit constant. When κ =K, we haveC= 1 and recover the Brock–Weinstock inequality, asserting that geodesic balls uniquely maximize the first nonzero Steklov eigenvalue among domains of the same volume, in Euclidean space and the hyperbolic space. 

   more » « less
3. Target signatures for thin surfaces

   https://doi.org/10.1088/1361-6420/ac4154  

   Cakoni, Fioralba; Monk, Peter; Zhang, Yangwen (January 2022, Inverse Problems)

   Abstract We investigate an inverse scattering problem for a thin inhomogeneous scatterer in R m , m = 2, 3, which we model as an m − 1 dimensional open surface. The scatterer is referred to as a screen. The goal is to design target signatures that are computable from scattering data in order to detect changes in the material properties of the screen. This target signature is characterized by a mixed Steklov eigenvalue problem for a domain whose boundary contains the screen. We show that the corresponding eigenvalues can be determined from appropriately modified scattering data by using the generalized linear sampling method. A weaker justification is provided for the classical linear sampling method. Numerical experiments are presented to support our theoretical results. 

   more » « less
4. Local version of Courant’s nodal domain theorem

   https://doi.org/10.4310/jdg/1707767334  

   Chanillo, Sagun; Logunov, Alexander; Malinnikova, Eugenia; Mangoubi, Dan (January 2024, Journal of Differential Geometry)

   Let (M,g) be a compact n-dimensional Riemannian manifold without boundary, where the metric g is C^1-smooth. Consider the sequence of eigenfunctions u_k of the Laplace operator on M. Let B be a ball on M. We prove that the number of nodal domains of u_k that intersect B is not greater than C_1Volume(B)k+C_2k^{(n-1)/n}, where C_1 and C_2 depend on (M,g) only. The problem of local bounds for the volume and for the number of nodal domains was raised by Donnelly and Fefferman, who also proposed an idea how one can prove such bounds. We combine their idea with two ingredients: the recent sharp Remez type inequality for eigenfunctions and the Landis type growth lemma in narrow domains. 

   more » « less
5. Min–max theory for free boundary minimal hypersurfaces II: general Morse index bounds and applications

   https://doi.org/10.1007/s00208-020-02096-0  

   Guang, Qiang; Li, Martin Man-chun; Wang, Zhichao; Zhou, Xin (April 2021, Mathematische Annalen)

   null (Ed.)

   Abstract For any smooth Riemannian metric on an $$(n+1)$$ ( n + 1 ) -dimensional compact manifold with boundary $$(M,\partial M)$$ ( M , ∂ M ) where $$3\le (n+1)\le 7$$ 3 ≤ ( n + 1 ) ≤ 7 , we establish general upper bounds for the Morse index of free boundary minimal hypersurfaces produced by min–max theory in the Almgren–Pitts setting. We apply our Morse index estimates to prove that for almost every (in the $$C^\infty $$ C ∞ Baire sense) Riemannan metric, the union of all compact, properly embedded free boundary minimal hypersurfaces is dense in M . If $$\partial M$$ ∂ M is further assumed to have a strictly mean convex point, we show the existence of infinitely many compact, properly embedded free boundary minimal hypersurfaces whose boundaries are non-empty. Our results prove a conjecture of Yau for generic metrics in the free boundary setting. 

   more » « less