Stability of reaction-fronts in porous media

The stability of reaction-fronts in porous media is studied with analytical and numerical methods. A stability criterion has been derived using linear stability analysis assuming a sharp font. The sharp front assumption is an approximation of the mathematical model in the limit of an infinite rapid reaction. The criterion shows that the stability of a sharp reaction front is dependent on the permeability that develops behind it. The sharp front is unstable for perturbations of any wave-length if the permeability increases behind the front. The criterion shows that short wave-length perturbations are more unstable than long wave-length perturbations. The sharp front is labile when the permeabilities are the same at both sides of the front. This means that the perturbed front moves unchanged forward. Finally, perturbations will die out in case the permeability decreases behind the sharp front. The stability of non-sharp fronts are simulated numerically when dissolution is by first order kinetics, the transport is by convection and diffusion and when the permeability and specific reactive surface depends on the porosity. The numerical experiments behave according to the stability criterion.     

本刊英文版Vol.31(2015),No.5论文摘要（英文）

<正>The Optimal Constant in Hardy-type Inequalities Mu-Fa CHEN Abstract To estimate the optimal constant in Hardy-type inequalities,some variational formulas and approximating procedures are introduced.The known basic estimates are improved considerably.The results are illustrated by typical examples.It is shown that the sharp factor is meaningful for each finite interval and a classical sharp model is re-examined.     

Hausdorff operators on the Heisenberg group

This paper is devoted to the high-dimensional and multilinear Hausdorff operators on the Heisenberg group H n. The sharp bounds for the strong type(p, p)(1 ≤ p ≤∞) estimates of n-dimensional Hausdorff operators on H n are obtained. The sharp bounds for strong(p, p) estimates are further extended to multilinear cases. As an application, we derive the sharp constant for the multilinear Hardy operator on H n. The weak type(p, p)(1 ≤ p ≤∞) estimates are also obtained.     

Nielsen numbers of mappings of surfaces

With each continuous map f of a compact polyhedron into itself there is associated a certain natural number, its Nielsen number N(f). The Nielsen number N(f) is bounded below by the number of fixed points of any map homotopic to f. The question of the sharpness of this estimate is classical: can one find for a given map f a map homotopic to it having exactly N(f) fixed points? It is known that this estimate is not sharp in general and that it is sharp for maps of compact polyhedra, which do not have locally separating points and which are not surfaces. The main result of the paper shows that this estimate is sharp for homotopy self-equivalence of compact surfaces. Its proof is based on Thurston's theory of diffeomorphisms of surfaces. In addition examples of maps of compact surfaces into themselves are discussed in the paper for which it seems that this estimate is not sharp.     

Sharp constants for Moser‐Trudinger inequalities on spheres in complex space ℂn

The main results of this paper concern sharp constants for the Moser‐Trudinger inequalities on spheres in complex space ?ⁿ. We derive Moser‐Trudinger inequalities for smooth functions and holomorphic functions with different sharp constants (see Theorem 1.1). The sharp Moser‐Trudinger inequalities under consideration involve the complex tangential gradients for the functions and thus we have shown here such inequalities in the CR setting. Though there is a close connection in spirit between inequalities proven here on complex spheres and those on the Heisenberg group for functions with compact support in any finite domain proven earlier by the same authors [17], derivation of the sharp constants for Moser‐Trudinger inequalities on complex spheres are more complicated and difficult to obtain than on the Heisenberg group. Variants of Moser‐Onofri‐type inequalities are also given on complex spheres as applications of our sharp inequalities (see Theorems 1.2 and 1.3). One of the key ingredients in deriving the main theorems is a sharp representation formula for functions on the complex spheres in terms of complex tangential gradients (see Theorem 1.4). © 2004 Wiley Periodicals, Inc.     

The behavior of the QR-factorization algorithm with column pivoting

A bound on the performance of QR-factorization with column pivoting is derived and two classes of matrices are constructed for which the bound is sharp or asymptotically sharp.     

Generalized weak sharp minima in cone-constrained convex optimization on Hadamard manifolds

In this paper, a convex optimization problem with cone constraint (for short, CPC) is introduced and studied on Hadamard manifolds. Some criteria and characterizations for the solution set to be a set of generalized global weak sharp minima, generalized local weak sharp minima and generalized bounded weak sharp minima for (CPC) are derived on Hadamard manifolds.     

The radius of convexity of particular functions and applications to the study of a second order differential inequality

In this paper we determine the radius of convexity of particular functions. The obtained results are used to deduce sharp estimates regarding functions which satisfy a second order differential subordination. A lemma regarding starlikeness that involves the notion of convolution is established, and is used in order to obtain a sharp starlikeness condition.     

Bahadur-Kiefer theorems for the product-limit process

In the random censorship from the right model, strong and weak limit theorems for Bahadur-Kiefer type processes based on the product-limit estimator are established. The main theorm is sharp and may be considered as a final result as far as this type of research is concerned. As a consequence of this theorem a sharp uniform Bahadur representation for product-limit quantiles is obtained.     

Weighted rearrangement inequalities for local sharp maximal functions

Several weighted rearrangement inequalities for uncentered and centered local sharp functions are proved. These results are applied to obtain new weighted weak-type and strong-type estimates for singular integrals. A self-improving property of sharp function inequalities is established.

     

Higher order optimal approximation of Csiszar's f-divergence

Here are established various sharp and nearly optimal probabilistic inequalities giving the high order approximation of Csiszar's f-divergence between two probability measures, which is the most essential and general tool for their comparison. The above are done through Taylor's formula, generalized Taylor–Widder's formula, an alternative recent expansion formula. Based on these we give many representation formulae of Csiszar's distance, then we estimate in all directions their remainders by using either the norms approach or the modulus of continuity way. Most of the last probabilistic estimates are sharp or nearly sharp, attained by basic simple functions.     

Logarithmic Sobolev trace inequality

A logarithmic Sobolev trace inequality is derived. Bounds on the best constant for this inequality from above and below are investigated using the sharp Sobolev inequality and the sharp logarithmic Sobolev inequality.

     

关于α-凸函数的一个子类的Fekete-Szeg(o)不等式

摘要：引入α-凸函数的一个子类M（α;A,B）,讨论了这个函数类上的Fekete-Szego不等式,得到了准确的结果,推广了一些作者的结果．最后,给出了不等式在利用Hadamard积定义的函数类上的两个应用．     

No Dimension-Independent Core-Sets for Containment Under Homothetics

This paper deals with the containment problem under homothetics which has the minimal enclosing ball (MEB) problem as a prominent representative. We connect the problem to results in classic convex geometry and introduce a new series of radii, which we call core-radii. For the MEB problem, these radii have already been considered from a different point of view and sharp inequalities between them are known. In this paper sharp inequalities between core-radii for general containment under homothetics are obtained. Moreover, the presented inequalities are used to derive sharp upper bounds on the size of core-sets for containment under homothetics. In the MEB case, this yields a tight (dimension-independent) bound for the size of such core-sets. In the general case, we show that there are core-sets of size linear in the dimension and that this bound stays sharp even if the container is required to be symmetric.     

An unknotting theorem for delta and sharp edge‐homotopy

Two spatial embeddings of a graph are said to be delta (resp. sharp) edge‐homotopic if they are transformed into each other by self delta (resp. sharp) moves and ambient isotopies. We show that any two spatial embeddings of a graph are delta (resp. sharp) edge‐homotopic if and only if the graph does not contain a subgraph which is homeomorphic to the theta graph or the disjoint union of two 1‐spheres, or equivalently G is homeomorphic to a bouquet. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Log-Sobolev inequalities and regions with exterior exponential cusps

We begin by studying certain semigroup estimates which are more singular than those implied by a Sobolev embedding theorem but which are equivalent to certain logarithmic Sobolev inequalities. We then give a method for proving that such log-Sobolev inequalities hold for Euclidean regions which satisfy a particular Hardy-type inequality. Our main application is to show that domains which have exterior exponential cusps, and hence have no Sobolev embedding theorem, satisfy such heat kernel bounds provided the cusps are not too sharp. Finally, we consider a rotationally invariant domain with an exponentially sharp cusp and prove that ultracontractivity breaks down when the cusp becomes too sharp.     

Antiblocking systems and PD-sets

Since the permutation decoding algorithm is more efficient the smaller the size of the PD-set, it is important for the applications to find small PD-sets. A lower bound on the size of a PD-set is given by Gordon. There are examples for PD-sets, but up to now there is no method known to find PD-sets. The question arises whether the Gordon bound is sharp. To handle this problem we introduce the notion of antiblocking system and we show that there are examples where the Gordon bound is not sharp.     

General estimate of the first eigenvalue on manifolds

Ten sharp lower estimates of the first non-trivial eigenvalue of Laplacian on compact Riemannian manifolds are reviewed and compared. An improved variational formula, a general common estimate, and a new sharp one are added. The best lower estimates are now updated. The new estimates provide a global picture of what one can expect by our approach.     

Sharp frequency bounds for eigenfunctions of the Ornstein–Uhlenbeck operator

We prove sharp bounds for the growth rate of eigenfunctions of the Ornstein–Uhlenbeck operator and its natural generalizations. The bounds are sharp even up to lower order terms and have important applications to geometric flows.     

The sharp Sobolev and isoperimetric inequalities split twice

This paper shows that each of the sharp (endpoint) Sobolev inequality and the isoperimetric inequality can be split into two sharp and stronger inequalities through either the 1-variational capacity or the 1-integral affine surface area. Furthermore, some related sharp analytic and geometric inequalities are also explored.