Compact Surfaces with Constant Gaussian Curvature in Product Spaces

We prove that the only compact surfaces of positive constant Gaussian curvature in \mathbbH²×\mathbbR{\mathbb{H}^{2}\times\mathbb{R}} (resp. positive constant Gaussian curvature greater than 1 in \mathbbS²×\mathbbR{\mathbb{S}^{2}\times\mathbb{R}}) whose boundary Γ is contained in a slice of the ambient space and such that the surface intersects this slice at a constant angle along Γ, are the pieces of a rotational complete surface. We also obtain some area estimates for surfaces of positive constant Gaussian curvature in \mathbbH²×\mathbbR{\mathbb{H}^{2}\times\mathbb{R}} and positive constant Gaussian curvature greater than 1 in \mathbbS²×\mathbbR{\mathbb{S}^{2}\times\mathbb{R}} whose boundary is contained in a slice of the ambient space. These estimates are optimal in the sense that if the bounds are attained, the surface is again a piece of a rotational complete surface.     

Distortion and topology

For a self mapping f: D→D of the unit disk in C which has finite distortion, we give a separation condition on the components of the set where the distortion is very large - say greater than a given constant - which implies that f still extends homeomorphically and quasisymmetrically to the boundary S = ?D. Thus f shares its boundary values with a quasiconformal mapping whose distortion we explicitly estimate in terms of the data. This condition, uniformly separated in modulus, allows the set where the distortion is large to accumulate on the entire boundary S, but it does not allow a component to run out to the boundary - a necessary restriction. The lift of a Jordan domain in a Riemann surface to its universal cover D is always uniformly separated in modulus, and this allows us to apply these results in the theory of Riemann surfaces to identify an interesting link between the support of the high distortion of a map between surfaces and their geometry - again with explicit estimates. As part of our investigations, we study mappings ?: S → S which are the germs of a conformal mapping and give good bounds on the distortion of a quasiconformal extension of ? to the disk D. We then extend these results to the germs of quasisymmetric mappings. These appear of independent interest and identify new geometric invariants.     

R13中混合型曲面保主曲率等距变形

本文研究了Minkowski空间R₁³曲面的等距变形问题.建立了R₁³中曲面的共形、等距等概念.推广了O.Bonnet和S.S.Chern关于欧氏空间的结论.对R₁³出现的新情况——曲面的中曲率梯度类光作了一定探讨,得出的主要结果为:非平坦的、允许保主曲率等距变形的曲面一定不是W-曲面.     

An optimal multivariate spline method of recovery of twice differentiable functions

A continuous quadratic polynomial spline of several variables is constructed. It solves the optimal recovery problem studied by V.F. Babenko, S.V. Borodachov, and D.S. Skorokhodov for the class of functions defined on a convex polytope in R ^d , whose second derivatives in any direction are uniformly bounded, and for a periodic analogue of this class. The information consists of the values and gradients of the function at some finite set of nodes in R ^d .     

Shortest paths on convex hypersurfaces of Riemannian spaces

A convex hypersurface in a Riemannian space M^m is part of the boundary of an m-dimensional locally convex set. It is established that there exists an intrinsic metric of such a hypersurface and it has curvature which is bounded below in the sense of A. D. Aleksandrov; curves with bounded variation of rotation in are shortest paths in M^m. For surfaces in R^m these facts are well known; however, the constructions leading to them are in large part inapplicable to spaces M^m. Hence approximations to by smooth equidistant (not necessarily convex) ones and normal polygonal paths, introduced (in the case of R³) by Yu. F. Borisov are used.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 66, pp. 114–132, 1976.     

Complete classification of parallel Lorentz surfaces in neutral pseudo hyperbolic 4-space

A Lorentz surface of an indefinite space form is called a parallel surface if its second fundamental form is parallel with respect to the Van der Waerden-Bortolotti connection. Such surfaces are locally invariant under the reflection with respect to the normal space at each point. Parallel surfaces are important in geometry as well as in general relativity since extrinsic invariants of such surfaces do not change from point to point. Recently, parallel Lorentz surfaces in 4D neutral pseudo Euclidean 4-space $ \mathbb{E}_2^4 $ \mathbb{E}_2^4 and in neutral pseudo 4-sphere S ₂⁴ (1) were classified in [14] and in [10], respectively. In this paper, we completely classify parallel Lorentz surfaces in neutral pseudo hyperbolic 4-space H ₂⁴ (−1). Our main result states that there are 53 families of parallel Lorentz surfaces in H ₂⁴ (−1). Conversely, every parallel Lorentz surface in H ₂⁴ (−1) is obtained from the 53 families. As an immediate by-product, we achieve the complete classification of all parallel Lorentz surfaces in 4D neutral indefinite space forms.     

The multidirectional Neumann problem in ℝ4

The Neumann problem as formulated in Lipschitz domains with L^p boundary data is solved for harmonic functions in any compact polyhedral domain of ℝ⁴ that has a connected 3-manifold boundary. Energy estimates on the boundary are derived from new polyhedral Rellich formulas together with a Whitney type decomposition of the polyhedron into similar Lipschitz domains. The classical layer potentials are thereby shown to be semi-Fredholm. To settle the onto question a method of continuity is devised that uses the classical 3-manifold theory of E. E. Moise in order to untwist the polyhedral boundary into a Lipschitz boundary. It is shown that this untwisting can be extended to include the interior of the domain in local neighborhoods of the boundary. In this way the flattening arguments of B. E. J. Dahlberg and C. E. Kenig for the H¹_at Neumann problem can be extended to polyhedral domains in ℝ⁴. A compact polyhedral domain in ℝ⁶ of M. L. Curtis and E. C. Zeeman, based on a construction of M. H. A. Newman, shows that the untwisting and flattening techniques used here are unavailable in general for higher dimensional boundary value problems in polyhedra.     

On functional which are orthogonally additive modulo Z

Let E be a real inner product space with dimension at least 2, D ? E, f: E → R with f(x+y)?f(x)?f(y) ∈ Z for all orthogonal x,y ∈ E, and f(D) ? (?γ,γ)+Z witn some real γ > 0. We prove that, under some additional assumptions, there are a unique linear functional A: E → R and a unique constant d ∈ R with f(x)?d∥x∥²?A(x) ∈ Z for x ∈ E. We also show some applications of this result to the determination of solutions F: E → C of the conditional equation: F(x+y) = F(x)F(y) for all orthogonal x,y ∈ E.     

Maximum Modulus Sets and Segre Convexity

Let E be a totally real, analytic, n-dimensional manifold, foliated by analytic interpolation submanifolds of codimension 1, in the analytic boundary of a Segre-convex domain in ℂⁿ. Given a canonical defining function of the boundary of Ω in a point 0 of E : Im z₁ +R[Re z₁, z′, z̄′]=0. If all the odd exponents in the decomposition of R in irreducible factors, at 0,are greater than 1 then R≥0 and E is locally contained in a maximum modulus set.     

Weingarten surfaces and nonlinear partial differential equations

The sine-Gordon equation has been known for a long time as the equation satisfied by the angle between the two asymptotic lines on a surface inR ³ with constant Gauss curvature –1. In this paper, we consider the following question: Does any other soliton equation have a similar geometric interpretation? A method for finding all the equations that have such an interpretation using Weingarten surfaces inR ³ is given. It is proved that the sine-Gordon equation is the only partial differential equation describing a class of Weingarten surfaces inR ³ and having a geometricso(3)-scattering system. Moreover, it is shown that the elliptic Liouville equation and the elliptic sinh-Gordon equation are the only partial differential equations describing classes of Weingarten surfaces inR ³ and having geometricso(3,C)-scattering systems.     

Quadrature surfaces as free boundaries

This paper deals with a free boundary porblem connected with the concept “quadrature surface”. Let Ω?R ⁿ be a bounded domain with aC ² boundary and μ a measure compactly supported in Ω. Then we say ?Ω is a quadrature surface with respect to μ if the following overdetermined Cauchy problem has a solution. $$\Delta u = - \mu in \Omega ,u = 0 and \frac{{\partial u}}{{\partial v}} = - 1 on \partial \Omega .$$ Applying simple techniques, we derive basic inequalities and show uniform boundedness for the set of solutions. Distance estimates as well as uniqueness results are obtained in special cases, e.g. we show that if ?Ω and ?D are two quadrature surfaces for a fixed measure μ and Ω is convex, thenD?Ω. The main observation, however, is that if ?Ω is a quadrature surface for μ≥0 andxε?Ω, then the inward normal ray to ?Ω atx intersects the convex hull of supp μ. We also study relations between quadrature surfaces and quadrature domains.D is said to be a quadrature domain with respect to a mesure μ if there is a solution to the following overdetermined Cauchy problem: $$\Delta u = 1 - \mu in D, andu = |\nabla u| = 0 on \partial D.$$ Finally, we apply our results to a problem of electrochemical machining.     

Zur lösungstheorie der zeitunabhängigen Maxwell-schen gleichungen mit der randbedingung n·b=n·d=0 in anisotropen,inhomogenen medien

A contribution to the theory of Maxwell's equation in the time-independent case with the boundary condition n·B=n·D=0 (n outer normal) for the interior and exterior problem of a bounded domain GR³ is given by means of Hilbert space methods. The problem is reduced to one with the boundary condition (in classical notation) n·curl E=n·curl H=0, which permits partial integration of the curl-operator. Carrying the existence and uniqueness theorems which are available in this case back to the original boundary value problem we get corresponding results.     

Finiteness of the set of solutions of Plateau's problem for polygonal boundary curves

It is proved that for a simple, closed, extreme polygon  Γ⊂R³$\Gamma \subset {\mathbb{R}}^3$ every immersed, stable minimal surface spanning Γ is an isolated point of the set of all minimal surfaces spanning Γ   w.r.t. the C⁰$C^0$-topology. Since the subset of immersed, stable minimal surfaces spanning Γ is shown to be closed in the compact set of all minimal surfaces spanning Γ, this proves in particular that Γ can bound only finitely many immersed, stable minimal surfaces.     

Capacity Theory for Monotone Operators

If Au = - div(a(x, Du)) is a monotone operator defined on the Sobolev space W^1,P(R ⁿ, 1 < p < + , with a(x,0) = 0 for a.e. x Rⁿ, the capacity C_A(E,F) relative to A can be defined for every pair (E,F) of bounded sets in Rⁿ with E F. The main properties of the set function C_A(E,F) are investigated. In particular it is proved that C_A(E,F) is increasing and countably subadditive with respect to E, decreasing with respect to F, and continuous, in a suitable sense, with respect to E and F.     

The Minding formula and its applications

We apply the Minding Formula for geodesic curvature and the Gauss-Bonnet Formula to calculate the total Gaussian curvature of certain 2-dimensional open complete branched Riemannian manifolds, the M\cal M surfaces. We prove that for an M\cal M surface, the total curvature depends only on its Euler characteristic and the local behaviour of its metric at ends and branch points. Then we check that many important surfaces, such as complete minimal surfaces in \Bbb Rⁿ{\Bbb R}^n with finite total curvature, complete constant mean curvature surfaces in hyperbolic 3-space H³ (–1) with finite total curvature, are actually branch point free M\cal M surfaces. Therefore as corollaries we give simple proofs of some classical theorems such as the Chern-Osserman theorem for complete minimal surfaces in \Bbb Rⁿ{\Bbb R}^n with finite total curvature. For the reader's convenience, we also derive the Minding Formula.     

A notion of robustness and stability of manifolds

Starting from the notion of thickness of Parks we define a notion of robustness for arbitrary subsets of Rk and we investigate its relationship with the notion of positive reach of Federer. We prove that if a set M is robust, then its boundary ∂M is of positive reach and conversely (under very mild restrictions) if ∂M is of positive reach, then M is robust. We then prove that a closed non-empty robust set in Rk (different from Rk) is a codimension zero submanifold of class C¹ with boundary. As a partial converse we show that any compact codimension zero submanifold with boundary of class C² is robust. Using the notion of robustness we prove a kind of stability theorem for codimension zero compact submanifolds with boundary: two such submanifolds, whose boundaries are close enough (in the sense of Hausdorff distance), are diffeomorphic.     

On rationalized H- and co-H-spaces with an appendix on decomposable H- and co-H-spaces

Let R be a subring of the rationals with 1/2, 1/3R; let S _R ⁿ denote the R-local n-sphere and define _R ⁿ :=S _R ⁿ for n odd, _R ⁿ :=S _R ⁿ for n>0 even. An H-space (resp. a 1-conn. co-H-space) is decomposable over R, if it is homotopy equivalent to a weak product of spaces _R ⁿ (resp. to a wedge of R-local spheres). We prove that, if E is grouplike decomposable of finite type over R, the functor [-,E] is determined on finite dim. complexes by the Hopf algebra M^*(E;R); here M^* denotes the unstable cohomotopy functor of H.J. Baues. If C is cogrouplike decomposable over R, the functor [C,-] is determined on 1-conn. R-local spaces by _*(C) as a cogroup in the category of M-Lie algebras. For R = the functor [-,E] is also determined by the Lie algebra _*(E) and [C,-] by the Berstein coalgebra associated to the comultiplication of C.     

Integrability of Green potentials in fractal domains

We proveL ^q -inequalities for the gradient of the Green potential (Gf) in bounded, connected NTA-domains inR ⁿ ,n≥2. These domains may have a highly non-rectifiable boundary and in the plane the set of all bounded simply connected NTA-domains coincides with the set of all quasidiscs. We get a restriction on the exponentq for which our inequalities are valid in terms of the validity of a reverse Hölder inequality for the Green function close to the boundary.     

The Dirichlet problem with a volume constraint

For B the open unit disk in R², let W₁(B) denote the Sobolev space of vector functions x: B→R³ such that x and its first partial derivatives are square integrable. For any y∈W₁(B), S(y) is the set of all x in W₁(B) for which x-y∈W₁₀(B), the closure in W₁(B) of C ₀ ^∞ (B). Assume that for all x ∈ S(y) the area functional A(x)>0. For a given constant K, we show that there is an x_o∈S(y) minimizing the “Dirichlet Integral” $$D(x) = \iint_B {(|x_u |^2 } + |x_v |^2 )dudv$$ in the subset of all x ∈ S(y) for which the oriented volume enclosed by y and x, V(y,x)=K. x_o is analytic on B and is a solution to the differential equation Δx=2H(x_u∧x_v) for some constant H.     

A decomposition theorem for χ1-convex sets

A set S in $\text{R}$ is said to be χ-convex if and only if S does not contain a visually independent subset having cardinality χ. It is natural to ask when an χ-convex set may be expressed as a countable union of convex sets. Here it is proved that if S is a closed χ-convex set in the plane and $\text{R}$ has at most finitely many bounded components, then S is a countable union of convex sets. A parallel result holds in $\text{R}$ when S is a closed χ-convex set which contains all triangular regions whose relative boundaries are in S. However, the result fails for arbitrary χ-convex sets, even in the plane.