Surjective partial differential operators on real analytic functions defined on open convex sets

Let P(D) be a partial differential operator with constant coefficients which is surjective on the space A(Ω) of real analytic functions on a covex open set Ω⊂ℝ ⁿ . Let L(P _m ) denote the localizations at ∞ (in the sense of H?rmander) of the principal part P _m . Then Q(x+iτN)≠ 0 for (x,τ)∈ℝ ⁿ ×(ℝ\{ 0}) for any Q∈L(P _m ) if N is a normal to δΩ which is noncharacteristic for Q. Under additional assumptions this implies that P _m must be locally hyperbolic. Received: 24 January 2000     

Complex<Emphasis Type="BoldItalic">n</Emphasis>-dimensional manifolds with a real<Emphasis Type="BoldItalic">n</Emphasis><Superscript>2</Superscript>-dimensional automorphism group

The main theorem of this article is a characterization of non compact simply connected complete Kobayashi hyperbolic complex manifold of dimension n≽ 2 with real n ²-dimensional holomorphic automorphism group. Together with the earlier work [11, 12] and [13] of Isaev and Krantz, this yields a complete classification of the simply-connected, complete Kobayashi hyperbolic manifolds with dim_ℝ Aut (M) ≽ (dim_ℂ M)².     

Estimates of the periods of periodic points for nonexpansive operators

Suppose thatE is a finite-dimensional Banach space with a polyhedral norm ‖·‖, i.e., a norm such that the unit ball inE is a polyhedron. ℝ ⁿ with the sup norm or ℝ ⁿ with thel ₁-norm are important examples. IfD is a bounded set inE andT:D→D is a map such that ‖T(y)−T(z)‖≤ ‖y−z‖ for ally andz inE, thenT is called nonexpansive with respect to ‖·‖, and it is known that for eachx ∈D there is an integerp=p(x) such that lim _j→∞ T ^jp (x) exists. Furthermore, there exists an integerN, depending only on the dimension ofE and the polyhedral norm onE, such thatp(x)≤N: see [1,12,18,19] and the references to the literature there. In [15], Scheutzow has raised a question about the optimal choice ofN whenE=ℝ ⁿ ,D=K ⁿ , the set of nonnegative vectors in ℝ ⁿ , and the norm is thel ₁-norm. We provide here a reasonably sharp answer to Scheutzow’s question, and in fact we provide a systematic way to generate examples and use this approach to prove that our estimates are optimal forn≤24. See Theorem 2.1, Table 2.1 and the examples in Section 3. As we show in Corollary 2.3, these results also provide information about the caseD=ℝ ⁿ , i.e.,T:ℝ ⁿ →ℝ ⁿ isl ₁-nonexpansive. In addition, it is conjectured in [12] thatN=2 ⁿ whenE=ℝ ⁿ and the norm is the sup norm, and such a result is optimal, if true. Our theorems here show that a sharper result is true for an important subclass of nonexpansive mapsT:(ℝ ⁿ ,‖ · ‖_∞)→(ℝ ⁿ ,‖ · ‖_∞). Partially supported by NSF DMS89-03018.     

Study of Some Fundamental Projective Quadrics Associated with a Standard Pseudo-Hermitian Space H p,q

This self-contained short note deals with the study of the properties of some real projective compact quadrics associated with a a standard pseudo-hermitian space H _p,q , namely [(Q(p, q))\tilde], [(Q_2p+1,1)\tilde], [(Q_1,2q+1)\tilde], [(H_p,q)\tilde].  [(Q(p, q))\tilde]{\widetilde{Q(p, q)}, \widetilde{Q_{2p+1,1}}, \widetilde{Q_{1,2q+1}}, \widetilde{H_{p,q}}. \, \widetilde{Q(p, q)}} is the (2n – 2) real projective quadric diffeomorphic to (S ^2p–1 × S ^2q–1)/Z ₂. inside the real projective space P(E ₁), where E ₁ is the real 2n-dimensional space subordinate to H _p,q . The properties of [(Q(p, q))\tilde]{\widetilde{Q(p, q)}} are investigated. [(H_p,q)\tilde]{\widetilde{H_p,q}} is the real (2n – 3)-dimensional compact manifold-(projective quadric)- associated with H _p,q , inside the complex projective space P(H _p,q ), diffeomorphic to (S ^2p–1 × S ^2q–1)/S ¹. The properties of [(H_p,q)\tilde]{\widetilde{H_{p,q}}} are studied. [(Q_2p+1,1)\tilde]{\widetilde{Q_{2p+1,1}}} is a 2p-dimensional standard real projective quadric, and [(Q_1,2q+1)\tilde]{\widetilde{Q_{1,2q+1}}} is another standard 2q-dimensional projective quadric. [(Q_2p+1,1)\tilde] è[(Q_1,2q+1)\tilde]{\widetilde{Q_{2p+1,1}} \cup \widetilde{Q_{1,2q+1}}}, union of two compact quadrics plays a part in the understanding of the "special pseudo-unitary conformal compactification" of H _p,q . It is shown how a distribution y → D _y , where y ? H\{0},H{y \in H\backslash\{0\},H} being the isotropic cone of H _p,q allows to [(H_p+1,q+1)\tilde]{\widetilde{H_{p+1,q+1}}} to be considered as a "special pseudo-unitary conformal compactified" of H _p,q × R. The following results precise the presentation given in [1,c].     

The Fourier transform of order statistics with applications to Lorentz spaces

We present a formula for the Fourier transforms of order statistics in ℝ ⁿ showing that all these Fourier transforms are equal up to a constant multiple outside the coordinate planes in ℝ ⁿ . Fora ₁≥...≥a _n≥0 andq>0, denote by ℓ _w,q ⁿ then-dimensional Lorentz space with the norm ‖(x ₁,...,x _n)‖=(a ₁(x ₁ ^* ) ^q +...+a _n(x _n ^* ) ^q )^1/q , where (x ₁ ^* ,...,x _n ^* ) is the non-increasing permutation of the numbers |x ₁|,...,|x _n|. We use the above mentioned formula and the Fourier transform criterion of isometric embeddability of Banach spaces intoL _q [10] to prove that, forn≥3 andq≤1, the space ℓ _w,q ⁿ is isometric to a subspace ofL _q if and only if the numbersa ₁,...,a _n form an arithmetic progression. Forq>1, all the numbersa _i must be equal so that ℓ _w,q ⁿ = ℓ _q ⁿ . Consequently, the Lorentz function spaceL _w,q(0, 1) is isometric to a subspace ofL _q if and only ifeither 0<q<∞ and the weightw is a constant function (so thatL _w,q=L_q),or q≤1 andw(t) is a decreasing linear function. Finally, we relate our results to the theory of positive definite functions. Both authors were supported in part by the NSF Workshop in Linear Analysis and Probability held at Texas A&M University in August 1993. The work was done during the first author’s visit to Texas A&M University.     

The classification of complete orientable 2-dimensional area-minimizing mod 2 surfaces in ℝn

Let S ⊂ ℝⁿ be a complete 2-dimensional areaminimizing mod 2 surface. Then S = x₁ (M₁) ∪ … ∪ x_r (M_r) where each M_j is connected, x_j: M_j → V_j is a classical minimal immersion into an affine subspace V_j of ℝⁿ, and the subspaces V₁,…, V_r are pairwise orthogonal. Here we prove that if M_j is orientable, then x_j (M_j) is either aflat plane or, in suitable coordinates, a generalized complex hyperbola.     

On Projections of Semi-Algebraic Sets Defined by Few Quadratic Inequalities

Let S⊂ℝ ^k+m be a compact semi-algebraic set defined by P ₁≥0,…,P _ℓ ≥0, where P _i ∈ℝ[X ₁,…,X _k ,Y ₁,…,Y _m ], and deg (P _i )≤2, 1≤i≤ℓ. Let π denote the standard projection from ℝ ^k+m onto ℝ ^m . We prove that for any q>0, the sum of the first q Betti numbers of π(S) is bounded by (k+m) ^{O(q ℓ)}. We also present an algorithm for computing the first q Betti numbers of π(S), whose complexity is . For fixed q and ℓ, both the bounds are polynomial in k+m. The author was supported in part by an NSF Career Award 0133597 and a Sloan Foundation Fellowship.     

Dynamical symmetry breaking in hyperbolic 4D spacetime and in extra dimensions

We study the dynamical symmetry breaking in quark matter within two different models. First, we consider the effect of gravitational catalysis of chiral and color symmetries breaking in strong gravitational field of ultrastatic hyperbolic spacetime ℝ ⊗ H ³ in the framework of an extended Nambu-Jona-Lasinio model. Second, we discuss the dynamical fermion mass generation in the flat 4-dimensional brane situated in the 5D spacetime with one extra dimension compactified on a circle. In the model, bulk fermions interact with fermions on the brane in the presence of a constant abelian gauge field A ₅ in the bulk. The influence of the A ₅-gauge field on the symmetry breaking is considered both when this field is a background parameter and a dynamical variable.     

Classification(s) of Danielewski hypersurfaces

The Danielewski hypersurfaces are the hypersurfaces X _Q,n in \mathbbC³ {\mathbb{C}^3} defined by an equation of the form x ⁿ y = Q(x, z) where n ⩾ 1 and Q(x, z) is a polynomial such that Q(0, z) is of degree at least two. They were studied by many authors during the last twenty years. In the present article, we give their classification as algebraic varieties. We also give their classification up to automorphism of the ambient space. As a corollary, we obtain that every Danielewski hypersurface X _Q,n with n ⩾ 2 admits at least two nonequivalent embeddings into \mathbbC³ {\mathbb{C}^3} .     

The Helmholtz decomposition of weightedLq-spaces for Muckenhoupt weights

We consider weights of Muckenhoupt classA _q, 1<q<∞. For a bounded Lipschitz domain Ω⊂ℝⁿ we prove a compact embedding and a Poincaré inequality in weighted Sobolev spaces. These technical tools allow us to solve the weak Neumann problem for the Laplace equation in weighted spaces on ℝⁿ, ℝⁿ ₊, on bounded and on exterior domains Ω with boundary of classC ¹, which will yield the Helmholtz decomposition ofL _ω ^q(Ω)ⁿ for general ω∈A _q. This is done by transferring the method of Simader and Sohr [4] to the weighted case. Our result generalizes a result of Farwig and Sohr [2] where the Helmholtz decomposition ofL _ω ^p(Ω)ⁿ is proved for an exterior domain and weights of Muckenhoupt class without singularities or degeneracies in a neighbourhood of ϖΩ.

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Sunto In questo lavoro consideriamo dei pesi della classe di MuckenhouptA _q, 1<q<∞. Per un dominio limitato lipschitziano Ω⊂ℝⁿ, dimostriamo una immersione compatta ed una disuguaglianza di Poincaré in spazi di Sobolev con peso. Questa tecnica ci consente di risolvere il problema debole di Neumann per l’equazione di Laplace in spazi pesati in ℝⁿ, ℝⁿ ₊ in domini limitati ed in domini esterni con frontiera di classeC ¹, che conduce alla decomposizione di Helmholtz diL _ω ^q(Ω)ⁿ per un qualsiasi ω∈A _q. Il risultato è ottenuto trasferendo il metodo di Simader e Sohr [4] al caso pesato. Quello qui presente estende un risultato di Farwig e Sohr [2] dove la decomposizione di Helmholtz diL _ω ^q(Ω)ⁿ è dimostrata per domini esterni e pesi della classe di Muckenhoupt privi di singolarità in un intorno di ϖΩ.

     

Extension of Floquet''''s theory to nonlinear quasiperiodic differential equations

In this paper, we consider the following autonomous system of differential equations: x = Ax f(x,θ), θ = ω, where θ∈Rm, ω = (ω1,…,ωm) ∈ Rm, x ∈ Rn, A ∈ Rn×n is a constant matrix and is hyperbolic, f is a C∞ function in both variables and 2π-periodic in each component of the vector e which satisfies f = O(||x||2) as x → 0. We study the normal form of this system and prove that under some proper conditions this system can be transformed to an autonomous system: x = Ax g(x), θ = ω. Additionally, the proof of this paper naturally implies the extension of Chen's theory in the quasi-periodic case.     

On a class of generalized Gon arov polynomials

The convergence properties of the class of Gon arov polynomials {Q_k(z; z₀,…, z_{k − 1})} generated through the qth derivative (q ≠ 1) are investigated in the present paper when z_k = a t^k, k 0, where a and t are any complex numbers. The investigations carried on here cover the possible ranges of variation of t and q, namely, ¦t¦>, =, < 1 when 0 q < 1, and ¦t¦ >, =, <1/q when q > 1. Except for the cases ¦t¦>1, q < 1, and ¦t¦>1/q, q > 1, the results obtained in the present work ensure the effectiveness of the set {Q_k(z)} in finite circles.     

Decompositions of the Hardy space Hz^1（Ω）

We give a decomposition of the Hardy space Hz^1（Ω） into ＂div-curl＂ quantities for Lipschitz domains in R^n. We also prove a decomposition of Hz^1（Ω） into Jacobians det Du, u ∈ W0^1,2 （Ω,R^2） for Ω in R^2. This partially answers a well-known open problem.     

Dini Set-Valued Directional Derivative in Locally Lipschitz Vector Optimization

The present paper studies the following constrained vector optimization problem: min  _C f(x), g(x)∈−K, h(x)=0, where f:ℝ ⁿ →ℝ ^m , g:ℝ ⁿ →ℝ ^p and h:ℝ ⁿ →ℝ ^q are locally Lipschitz functions and C⊂ℝ ^m , K⊂ℝ ^p are closed convex cones. In terms of the Dini set-valued directional derivative, first-order necessary and first-order sufficient conditions are obtained for a point x ⁰ to be a w-minimizer (weakly efficient point) or an i-minimizer (isolated minimizer of order 1). It is shown that, under natural assumptions (given by a nonsmooth variant of the implicit function theorem for the equality constraints), the obtained conditions improve some given by Clarke and Craven. Further comparison is done with some recent results of Khanh, Tuan and of Jiiménez, Novo.     

Multidimensional version of M. A. Krasnoseľskii’s generalized contraction principle

Let M be a complete K-metric space with n-dimensional metric ρ(x, y): M × M → R ⁿ , where K is the cone of nonnegative vectors in R ⁿ . A mapping F: M → M is called a Q-contraction if ρ (Fx,Fy) ⩽ Qρ (x,y), where Q: K → K is a semi-additive absolutely stable mapping. A Q-contraction always has a unique fixed point x* in M, and ρ(x*,a) ⩽ (I - Q)^-1 ρ(Fa, a) for every point a in M. The point x* can be obtained by the successive approximation method x _k = Fx _k-1, k = 1, 2,..., starting from an arbitrary point x ₀ in M, and the following error estimates hold: ρ (x*, x _k ) ⩽ Q ^k (I - Q)^-1ρ(x ₁, _x ⁰) ⩽ (I - Q)^-1 Q ^k ρ(x ₁, x ₀), k = 1, 2,.... Generally the mappings (I - Q)^-1 and Q ^k do not commute. For n = 1, the result is close to M. A. Krasnosel’skii’s generalized contraction principle.     

Ann-dimensional search problem with restricted questions

The problem is the following: How many questions are necessary in the worst case to determine whether a pointX in then-dimensional Euclidean spaceR ⁿ belongs to then-dimensional unit cubeQ ⁿ, where we are allowed to ask which halfspaces of (n−1)-dimensional hyperplanes contain the pointX? It is known that ⌌3n/2⌍ questions are sufficient. We prove here thatcn questions are necessary, wherec≈1.2938 is the solution of the equationx log₂ x−(x−1) log₂ (x−1)=1.     

Three-Dimensional Polyhedra Can Be Described by Three Polynomial Inequalities

Bosse et al. conjectured that for every natural number d≥2 and every d-dimensional polytope P in ℝ ^d , there exist d polynomials p ₁(x),…,p _d (x) satisfying P={x∈ℝ ^d :p ₁(x)≥0,…,p _d (x)≥0}. We show that every three-dimensional polyhedron can be described by three polynomial inequalities, which confirms the conjecture for the case d=3 but also provides an analogous statement for the case of unbounded polyhedra. The proof of our result is constructive. Work supported by the German Research Foundation within the Research Unit 468 “Methods from Discrete Mathematics for the Synthesis and Control of Chemical Processes”.     

A homological characterization of hyperbolic groups

A finitely presented group G is hyperbolic iff H ⁽¹⁾ ₁(G,ℝ)=0=⁽¹⁾ ₂(G, ℝ), where H ⁽¹⁾ _* (resp. ⁽¹⁾ _*) denotes the ℓ₁-homology (resp. reduced ℓ₁-homology). If Γ is a graph, then every ℓ₁ 1-cycle in Γ with real coefficients can be approximated by 1-cycles of compact support. A 1-relator group G is hyperbolic iff H ⁽¹⁾ ₁(G,ℝ)=0. Oblatum: 30-IV-1997 & 14-V-1998 / Published online: 14 January 1999     

L P boundedness of area function associated with operators on product spaces

Let L be the infinitesimal generator of an analytic semigroup on L2 (Rn) with suitable upper bounds on its heat kernels. Assume that L has a bounded holomorphic functional calculus on L2(Rn). In this paper,we define the Littlewood- Paley g function associated with L on Rn × Rn, denoted by GL(f)(x1, x2), and decomposition, we prove that ‖SL(f)‖p ≈‖GL(f)‖p ≈‖f‖p for 1 ＜ p ＜∞.     

Deconstructing functions on quadratic surfaces into multipoles

Any homogeneous polynomial P(x, y, z) of degree d, being restricted to a unit sphere S ², admits essentially a unique representation of the form