Singular integrals associated to the Laplacian on the affine groupax+b

We consider singular integral operators of the form (a)Z ₁L^?1Z₂, (b)Z ₁Z₂L^?1, and (c)L ^?1Z₁Z₂, whereZ ₁ andZ ₂ are nonzero right-invariant vector fields, andL is theL ²-closure of a canonical Laplacian. The operators (a) are shown to be bounded onL ^p for allp∈(1, ∞) and of weak type (1, 1), whereas all of the operators in (b) and (c) are not of weak type (p, p) for anyp∈[1, ∞).     

The failure of the Hardy inequality and interpolation of intersections

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(w₀), N∩L_p(w₁)), whereN is the linear space which consists of all functions with the integral equal to 0. We calculate theK-functional for the couple (N∩L _p(w₀),N∩L _p (w₁)), which turns out to be essentially different from theK-functional for (L _p(w₀), L_p(w₁)), even for the case whenN∩L _p(w_i) is dense inL _p(w_i) (i=0,1). This essential difference is the reason why the “naive” interpolation above gives an incorrect result.     

L p -methods for robust regression

In this paper we discuss algorithms forL _p-methods, i.e. minimizers of theL _p-norm of the residual vector. The statistical “goodness” of the different methods when applied to regression problems is compared in a Monte Carlo experiment.     

The generalized analytic signal

This paper is an explication of the analytic signal in the generalized case, i.e., the analytic signal of a generalized function and of a generalized stochastic process. The contributions of the author are: (1) an L²-theory of distributions which, in the study of the analytic signal, has an advantage over the usual Schwartz-Itô-Gel'fand theory because the Cauchy representation is defined; (2) a proof (Theorem 2.5) that the Schwartz distributions δ, δ⁺, δ^? and ? may be extended to the L₂-case, expressions (Theorems 2.6 and 2.7) for their Hilbert and Fourier transforms in the L₂-case, and expressions (Section 2.1) for their analytic signals; (3) a proof (Theorem 3.3) that an orthogonal L₂-process, and therefore the Fourier transform of a second-order stationary stochastic process (Theorem 3.4), is strictly generalized; (4) a representation theorem (Theorem 3.5) which extends the Itô spectral representation theorem for stationary random distributions to the nonspectral, nonstationary, L₂-case; (5) expressions for the Cauchy representation (Theorem 3.6) and the analytic signal (Theorem 3.7) of an L₂-process; (6) an expression for and the covariance kernel of the analytic signal of white noise (Section 3.4). The word application in the text refers to the application of previously developed concepts.     

A limit theorem for order preserving nonexpansive operators inL 1

We prove the following theorem:Let T be an order preserving nonexpansive operator on L ₁ (μ) (or L ₁ ⁺ ) of a σ-finite measure, which also decreases theL _∞-norm, and let S=tI+(1?t)T for 0<t<1. Then for everyf ∈ L_p (1<p<∞),the sequence S ⁿf converges weakly in L_p. (The assumptions do not imply thatT is nonexpansive inL _p for anyp>1, even ifμ is finite.) For the proof we show that ∥S ⁿ⁺¹ f?S ⁿf∥ _p → 0 for everyf ∈L _p, 1<p<∞, and apply toS the following theorem:Let T be order preserving and nonexpansive in L ₁ ⁺ , and assume that T decreases theL _∞-norm. Then forg ∈L _p (1<p<∞) Tⁿg is weakly almost convergent. If forf ∈ L_p we have T ⁿ⁺¹ f?T ⁿ f → 0weakly, then T ⁿf converges weakly in L_p (1<p<∞).     

BMO- andL p -conditions for power series and Dirichlet series with positive coefficients

It is known that theL _p -norms of the sums of power series can be estimated from below and above by means of their coefficients, provided these coefficients are nonnegative. In the paper we prove analogous estimates for theL _p -norms of the sums of Dirichlet series. Our main result gives exact lower and upper estimates for the BMO-norm of the sums of power series and Dirichlet series, respectively, by means of their coefficients.     

Jackson and smoothness theorems for freud weights

We prove Jackson, realization, and converse theorems for Freud weights inL _p, 0<p ≤ ∞. Even forp ≥ 1, our conditions on the weight in our Jackson theorems are far less restrictive than those previously imposed. Moreover, the method—of first approximating by a spline, and then by a polynomial—is new in this context, and of intrinsic interest, since it avoids the use of orthogonal polynomials for Freud weights. We establish some properties of the modulus of smoothness, valid inL _p for 0<p ≤ ∞. Since theK-functional is identically zero inL _p,p<1, the analysis of the modulus of continuity involves a different tool, namely, realization, which works inL _p for all 0<p ≤ ∞. We deduce Marchaud-type inequalities.     

Nonlinear diffusions, hypercontractivity and the optimal L-Euclidean logarithmic Sobolev inequality

The equation u_t=Δ_p(u^1/(p−1)) for p>1 is a nonlinear generalization of the heat equation which is also homogeneous, of degree 1. For large time asymptotics, its links with the optimal L^p-Euclidean logarithmic Sobolev inequality have recently been investigated. Here we focus on the existence and the uniqueness of the solutions to the Cauchy problem and on the regularization properties (hypercontractivity and ultracontractivity) of the equation using the L^p-Euclidean logarithmic Sobolev inequality. A large deviation result based on a Hamilton-Jacobi equation and also related to the L^p-Euclidean logarithmic Sobolev inequality is then stated.     

Variational problem with degeneracy at the boundary of the interval

The weighted L _p-norms of derivatives are estimated in terms of the weighted L _p-norm of the highest derivative and the traces of the function and its derivatives at the given points of closure of the bounded interval; weights are powers of the distance to the nearest endpoint of the interval. For functions with zero traces, sharper estimates are established. For the integral quadratic functional with degenerate coefficients, we prove the existence and uniqueness of the solution to the problem of minimization of a functional on a function class with zero traces.     

Quadrature error of the load vector in the finite element method

We analyze the error in thep version of the of the finite element method when the effect of the quadrature error is taken in the load vector. We briefly study some results on theH ¹ norm error and present some new results for the error in theL ₂ norm. We investigate the quadrature error due to the numerical integration of the right hand side We present theoretical and computational examples showing the sharpness of our results.     

Vanishing viscosity in the plane for vorticity in borderline spaces of Besov type

The existence and uniqueness of solutions to the Euler equations for initial vorticity in BΓ∩Lp₀∩Lp₁ was proved by Misha Vishik, where BΓ is a borderline Besov space parameterized by the function Γ and 1<p₀<2<p₁. Vishik established short time existence and uniqueness when Γ(n)=O(logn) and global existence and uniqueness when . For initial vorticity in BΓ∩L², we establish the vanishing viscosity limit in L²(R²) of solutions of the Navier-Stokes equations to a solution of the Euler equations in the plane, convergence being uniform over short time when Γ(n)=O(logn) and uniform over any finite time when Γ(n)=O(logκn), 0?κ<1, and we give a bound on the rate of convergence. This allows us to extend the class of initial vorticities for which both global existence and uniqueness of solutions to the Euler equations can be established to include BΓ∩L² when Γ(n)=O(logκn) for 0<κ<1.     

Discriminating potentials of measures on certain quasi-normed spaces

A uniqueness theorem for a convolution equation is proved for a class of infinitedimensional spaces larger than the class of Banach spaces, in particular, for L_p-spaces with p > 0.     

Die EindeutigkeitL 2-optimaler Polynomialer Monosplines

Summary In 1968 Schoenberg announced a result concerning unicity ofL ₂-optimal polynomial monosplines [26]. It is the aim of this paper to prove Schoenberg's statement.     

The homogeneous dirichlet problem for quasilinear anisotropic degenerate parabolic-hyperbolic equation with L initial value

The aim of this paper is to prove the well-posedness (existence and uniqueness) of the L^p entropy solution to the homogeneous Dirichlet problems for the anisotropic degenerate parabolic-hyperbolic equations with L^p initial value. We use the device of doubling variables and some technical analysis to prove the uniqueness result. Moreover we can prove that the L^p entropy solution can be obtained as the limit of solutions of the corresponding regularized equations of nondegenerate parabolic type.     

Some L p and Hölder Estimates for Divergence Type Nonlinear SPDEs on C 1-Domains

We prove uniqueness and existence of certain nonlinear stochastic partial differential equations (SPDEs) of divergence type defined on C ¹-domains. Some L _p and Hölder estimates of the solution and its gradient are also obtained.     

On the optimal quadrature least L∞-norm of monosplines with free knots on the real axis

In this paper, the exact solutions of the L_∝ norm on some classes of monosplines with free knots on the real axis are obtained, and the optimal quadrature formula and optimal error on the convolution class taking a PF density as its kernal are given.     

L p -spectral estimation with anL ∞-upper bound

Given the constraint 0≤f≤B, whereB is in the interior of the positive cone ofL _∞, and given a finite number of correlations off, we wish to estimatef. Since only a finite number of correlations are given, this does not uniquely determinef. We estimatef by picking the unique function Φ₀ satisfying the constraints and minimizing theL _p -norm with 1<p<∞. Under suitable conditions, the form of the solution is shown to be $$\Phi _0 (f) = \min \{ B(x), \max \{ 0,P(x)\} ^{1/(p - 1)} \} ,$$ whereP is a linear combination of the correlation functions.     

Smooth points ofp-adic subanalytic sets

Ap-adic subanalytic set shares with a real subanalytic set the fundamental property that its singular locus is itself subanalytic. Furthermore, given ap-adic subanalytic function ? with domain contained in ? _p ^m , there is an integerL such that for any pointx ₀ ∈ ? _p ^m in a neighborhood of whichf is defined,f has a Taylor approximation up to orderL atx ₀ if, and only if, ? is analytic aroundx ₀. These results extend to thep-adic fields real variables theorems by M. Tamm [21].     

Dual semigroups and second order linear elliptic boundary value problems

It is shown that general second order elliptic boundary value problems on bounded domains generate analytic semigroups onL ₁. The proof is based on Phillips’ theory of dual semigroups. Several sharp estimates for the corresponding semigroups inL _p, 1≦p<∞, are given.