Variations of Rado's lemma

The deductive strengths of three variations of Rado's selection lemma are studied in set theory without the axiom of choice. Two are shown to be equivalent to Rado's lemma and the third to the Boolean prime ideal theorem. MSC: 03E25, 04A25, 06E05.     

On local non‐compactness in recursive mathematics

A metric space is said to be locally non‐compact if every neighborhood contains a sequence that is eventually bounded away from every element of the space, hence contains no accumulation point. We show within recursive mathematics that a nonvoid complete metric space is locally non‐compact iff it is without isolated points. The result has an interesting consequence in computable analysis: If a complete metric space has a computable witness that it is without isolated points, then every neighborhood contains a computable sequence that is eventually computably bounded away from every computable element of the space. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Weak Hopf lemma for the invariant Laplacian and related elliptic operators

We obtain a weak version of the Hopf lemma for the invariant Laplacian on the unit ball of the complex n$n$-space. We also show that our result is sharp in some sense. Motivated by this result, we also consider a class of degenerate elliptic operators with the degeneracy depending on the distance to the boundary of the domain. We study the dependence of the validity of Hopf lemma on the degree of degeneracy of the operator. We show that Hopf lemma holds if the degeneracy is small and fails in general if the degeneracy is large. What is more interesting is the critical case for which we show that certain weak version of Hopf lemma holds.     

On proximal convergence in uniform spaces

The paper deals with proximal convergence and Leader's theorem, in the constructive theory of uniform apartness spaces. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dynamic evaluation of integrity and the computational content of Krull's lemma

A multiplicative subset of a commutative ring contains the zero element precisely if the set in question meets every prime ideal. While this form of Krull's Lemma takes recourse to transfinite reasoning, it has recently allowed for a crucial reduction to the integral case in Kemper and the third author's novel characterization of the valuative dimension. We present a dynamical solution by which transfinite reasoning can be avoided, and illustrate this constructive method with concrete examples. We further give a combinatorial explanation by relating the Zariski lattice to a certain inductively generated class of finite binary trees. In particular, we make explicit the computational content of Krull's Lemma.     

The swap of integral and limit in constructive mathematics

Integration within constructive, especially intuitionistic mathematics in the sense of L. E. J. Brouwer, slightly differs from formal integration theories: Some classical results, especially Lebesgue's dominated convergence theorem, have tobe substituted by appropriate alternatives. Although there exist sophisticated, but rather laborious proposals, e.g. by E. Bishop and D. S. Bridges (cf. [2]), the reference to partitions and the Riemann‐integral, also with regard to the results obtained by R. Henstock and J. Kurzweil (cf. [9], [12]), seems to give a better direction. Especially, convergence theorems can be proved by introducing the concept of “equi‐integrability”. The paper is strongly motivated by Brouwer's result that each function fully defined on a compact interval has necessarily to be uniformly continuous. Nevertheless, there are, with only one exception (a corollary of Theorem 4.2), no references to the fan‐theorem or to bar‐induction. Therefore, the whole paper can be read within the setting of Bishop's access to constructive mathematics. Nothing of genuine full‐fledged Brouwerian intuitionism is used for the main results in this note (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Appell’s lemma and conservation laws of KdV equation

An alternative method to construct a class of conservation laws of the KdV equation based on the classical Appell’s lemma and the trace formulas of Deift-Trubowitz type is studied. A new type of infinite sequence of conservation laws whose local densities cannot be expressed in terms of differential polynomials is constructed.     

A note on Arzela–Ascoli's lemma in almost periodic problems

This paper is concerned with a generalized Arzela–Ascoli's lemma, which has been extensively applied in almost periodic problems by the continuation theorem of degree theory. We give a counter example to show that this lemma is incorrect, and there is a gap in the proof of some existing literature, where the addressed generalized Arzela–Ascoli's lemma was used. Moreover, we make some final comments and introduce an open problem. Copyright © 2016 John Wiley & Sons, Ltd.     

The infinitesimal rigid displacement lemma in Lipschitz co‐ordinates and application to shells with minimal regularity

We establish a version of the infinitesimal rigid displacement lemma in curvilinear Lipschitz co‐ordinates. We give an application to linearly elastic shells whose midsurface and normal vector are both Lipschitz. Copyright © 2004 John Wiley Sons, Ltd.     

On regularities and Fredholm theory

We investigate the relationship between the regularities and the Fredholm theory in a Banach algebra.     

Phelps' lemma, Danes' drop theorem and Ekeland's principle in locally convex spaces

A generalization of Phelps' lemma to locally convex spaces is proven, applying its well-known Banach space version. We show the equivalence of this theorem, Ekeland's principle and Danes' drop theorem in locally convex spaces to their Banach space counterparts and to a Pareto efficiency theorem due to Isac. This solves a problem, concerning the drop theorem, proposed by G. Isac in 1997.

We show that a different formulation of Ekeland's principle in locally convex spaces, using a family of topology generating seminorms as perturbation functions rather than a single (in general discontinuous) Minkowski functional, turns out to be equivalent to the original version.

     

Compactness in spaces of inner regular measures and a general Portmanteau lemma

This paper may be understood as a continuation of Topsoe's seminal paper [F. Topsoe, Compactness in spaces of measures, Studia Math. 36 (1970) 195-212] to characterize, within an abstract setting, compact subsets of finite inner regular measures w.r.t. the weak topology. The new aspect is that neither assumptions on compactness of the inner approximating lattices nor nonsequential continuity properties for the measures will be imposed. As a providing step also a generalization of the classical Portmanteau lemma will be established. The obtained characterizations of compact subsets w.r.t. the weak topology encompass several known ones from literature. The investigations rely basically on the inner extension theory for measures which has been systemized recently by König [H. König, Measure and Integration, Springer, Berlin, 1997; H. König, On the inner Daniell-Stone and Riesz representation theorems, Doc. Mat. 5 (2000) 301-315; H. König, Measure and integration: An attempt at unified systematization, Rend. Istit. Mat. Univ. Trieste 34 (2002) 155-214].     

Common properties of bounded linear operators AC and BA: Local spectral theory

Let X, Y   be Banach spaces, A:X→Y$A:X\to Y$ and B,C:Y→X$B,C:Y\to X$ be bounded linear operators satisfying operator equation ABA=ACA$ABA=ACA$. In this paper, we show that AC and BA share the local spectral properties such as Bishop's property (β), subscalarity and Dunford's property (C). Also, the quasi-nilpotent part and the analytic core for AC and BA are studied.