In this paper we prove several results for the scattering phase (spectral shift function) related with perturbations of the electromagnetic field for the Dirac operator in the Euclidean space.

In this paper we study the ($L$^{$p$}($u$^{$p$}),$L$^{$q$}($v$^{$q$})) boundedness of the higher order commutators$T$_{$Ω,α,b$}^{$m$}and$M$_{$Ω,α,b$}^{$m$}formed by the fractional integral operator$T$_{$Ω,α$}, the fractional maximal operator$M$_{$Ω,α$}, and a function$b(x)$in BMO($v$), respectively.

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We study the solvability problem for the multidimensional Riccati equation −∇$u$=|∇$u$|^{q}+ω, where$q$>1 and ω is an arbitrary nonnegative function (or measure). We also discuss connections with the classical problem of the existence of positive solutions for the Schrödinger equation −Δ$u$−ω$u$=0 with nonnegative potential ω. We establish explicit criteria for the existence of global solutions on$R$^{n}in terms involving geometric (capacity) estimates or pointwise behavior of Riesz potentials, together with sharp pointwise estimates of solutions and their gradients. We also consider the corresponding nonlinear Dirichlet problem on a bounded domain, as well as more general equations of the type$−Lu=f(x, u, ∇u)$+ω where, and$L$is a uniformly elliptic operator.

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We consider the Newton equation(*) $$\ddot x = F\left( x \right), F\left( x \right) = - \nabla \upsilon \left( x \right), x \in R^d ,$$ for |$j$|≤2 and some α>1.

Let ϕ be an analytic function defined on the unit disk$D$, with ϕ($D$)⊂$D$, ϕ(0)=0, and ϕ′(0)=λ≠0. Then by a classical result of G. Kœnigs, the sequence of normalized iterates Φ_{$n$}/λ^{$n$}converges uniformly on compact subsets of$D$to a function σ analytic in$D$which satisfies$σ$°φ=λ$σ$. It is of interest in the study of composition operators to know if, whenever σ belongs to a Hardy space$H$_{$p$}, the sequence Φ_{$n$}/λ^{$n$}converges to σ in the norm of$H$_{$p$}. We show that this is indeed the case, generalizing a result of P. Bourdon obtained under the assumption that ϕ is univalent.

Let$I$be a union of finitely many closed intervals in [−1, 0). Let$I$^{↞}be a single interval of the form [−1, −a] chosen to have the same logarithmic length as$I$. Let$D$be the unit disc. Then, Beurling [8] has shown that the harmonic measure of the circle ∂$D$at the origin in the slit disc$D$/$I$is increased if$I$is replaced by$I$^{↞}. We prove a number of cognate results and extensions. For instance, we show that Beurling's result remains true if the intervals in$I$are not just one-dimensional, but if they in fact constitute polar rectangles centred on the negative real axis and having some fixed constant angular width. In doing this, we obtain a new proof of Beurling's result. We also discuss a conjecture of Matheson and Pruss [25] and some other open problems.

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No abstract uploaded!

It is shown that the punctual quotient scheme$Q$_{l}^{r}parametrizing all zero-dimensional quotients $$\mathcal{O}_{A^2 }^{ \oplus ^r } \to T$$ of length$l$and supported at some fixed point O∈$A$^{2}in the plane is irreducible.

This paper examines the boundary behaviour of superharmonic functions on a half-space in terms of their behaviour along lines normal to the boundary. It is shown that, if the set of lines along which such functions grow quickly is (in a certain sense) metrically dense, then the set of lines along which they are bounded below is topologically small.

We construct singular solutions to equations $$div\mathcal{A}(x,\nabla u) = 0,$$ similar to the$p$-Laplacian, that tend to ∞ on a given closed set of$p$-capacity zero. Moreover, we show that every$G$_{δ}-set of vanishing$p$-capacity is the infinity set of some$A$-superharmonic function.

We study removable sets for the Sobolev space$W$^{1,p}. We show that removability for sets lying in a hyperplane is essentially determined by their thickness measured in terms of a concept of$p$-porosity.

The equation$x$^{(n)}(t)=(−1)^{$n$}│$x(t)$│^{$k$}with$k$>1 is considered. In the case$n$≦4 it is proved that solutions defined in a neighbourhood of infinity coincide with$C$(t−t_{0})^{−n/(k−1)}, where$C$is a constant depending only on$n$and$k$. In the general case such solutions are Kneser solutions and can be estimated from above and below by a constant times ($t−t$_{0})^{−n/(k−1)}. It is shown that they do not necessarily coincide with$C$(t−t_{0})^{−n/(k−1)}. This gives a negative answer to two conjectures posed by Kiguradze that Kneser solutions are determined by their value in a point and that blow-up solutions have prescribed asymptotics.

The main idea of this paper is to clarify why it is sometimes incorrect to interpolate inequalities in a “formal” way. For this we consider two Hardy type inequalities, which are true for each parameter α≠0 but which fail for the “critical” point α=0. This means that we cannot interpolate these inequalities between the noncritical points α=1 and α=−1 and conclude that it is also true at the critical point α=0. Why? An accurate analysis shows that this problem is connected with the investigation of the interpolation of intersections ($N$∩$L$_{p}(w_{0}), N∩L_{p}(w_{1})), where$N$is the linear space which consists of all functions with the integral equal to 0. We calculate the$K$-functional for the couple ($N$∩$L$_{p}(w_{0}),$N$∩$L$_{p}(w_{1})), which turns out to be essentially different from the$K$-functional for ($L$_{p}(w_{0}), L_{p}(w_{1})), even for the case when$N$∩$L$_{p}(w_{i}) is dense in$L$_{p}(w_{i}) ($i$=0,1). This essential difference is the reason why the “naive” interpolation above gives an incorrect result.

A solution to a Dirichlet problem for the complex Monge-Ampère operator is characterized as a minimizer of an energy functional. A mutual energy estimate and a generalization of Hölder's inequality is proved. A comparison is made with corresponding results in classical potential theory.

We prove rigidity results for a class of non-uniformly hyperbolic holomorphic maps. If a holomorphic Collet-Eckmann map$f$is topologically conjugate to a holomorphic map$g$, then the conjugacy can be improved to be quasiconformal. If there is only one critical point in the repeller, then$g$is Collet-Eckmann, too.

Let Ω⊂$R$^{$n$}be an arbitrary open set. In this paper it is shown that if a Sobolev function$f$∈$W$^{1,$p$}(Ω) possesses a zero trace (in the sense of Lebesgue points) on ϖΩ, then$f$is weakly zero on ϖΩ in the sense that$f$∈$W$_{0}^{1,$p$}(Ω).

For Ξ∈$R$^{$n$},$t$∈$R$and$f$∈$S$($R$^{$n$}) define $$\left( {S^2 f} \right)\left( t \right)\left( \xi \right) = \exp \left( {it\left| \xi \right|^2 } \right)\hat f\left( \xi \right)$$ . We determine the optimal regularity$s$_{0}such that $$\int_{R^n } {\left\| {(S^2 f)[x]} \right\|_{L^2 (R)}^2 \frac{{dx}}{{(1 + |x|)^b }} \leqslant C\left\| f \right\|_{H^s (R^n )}^2 ,s > s_0 } ,$$ holds where$C$is independent of$f$∈$S$($R$^{$n$}) or we show that such optimal regularity does not exist. This problem has been treated earlier, e.g. by Ben-Artzi and Klainerman [2], Kato and Yajima [4], Simon [6], Vega [9] and Wang [11].

Let$M$be a connected, noncompact, complete Riemannian manifold, consider the operator$L=Δ+∇V$for some$V∈C$^{2}(M) with exp[$V$] integrable with respect to the Riemannian volume element. This paper studies the existence of the spectral gap of$L$. As a consequence of the main result, let ϱ be the distance function from a point o, then the spectral gap exists provided lim_{ϱ→∞}sup$L$_{ϱ<0}while the spectral gap does not exist if o is a pole and lim_{ϱ→∞}inf$L$_{ϱ≥0}. Moreover, the elliptic operators on$R$^{$d$}are also studied.

Let Δ be the closed unit disk in C, let Γ be the circle, let Π: Δ×C→Δ be projection, and let$A(Δ)$be the algebra of complex functions continuous on Δ and analytic in int Δ. Let$K$be a compact set in C^{2}such that Π($K$)=Γ, and let$K$_{λ}≠{w∈C|(λ,w)∈K}. Suppose further that (a) for every λ∈Γ,$K$_{λ}is the union of two nonempty disjoint connected compact sets with connected complement, (b) there exists a function Q(λ,w)≠(w-R(λ))^{2}-S(λ) quadratic in w with$R,S∈A(Δ)$such that for all λ∈Γ, {w∈C|Q(λ,w)=0}υ int$K$_{λ}, where$S$has only one zero in int Δ, counting multiplicity, and (c) for every λ∈Γ, the map ω→Q(λ,ω) is injective on each component of$K$_{λ}. Then we prove that К/K is the union of analytic disks 2-sheeted over int Δ, where К is the polynomial convex hull of$K$. Furthermore, we show that БК/K is the disjoint union of such disks.