Generalizations of theorems of J. Aczél and R. Ger and T. Kochanek

Periodica Mathematica Hungarica - We present a result which generalizes a theorem proved by Aczél (Bull Am Math Soc 54:392–400, 1948). Hence we obtain generalizations of a theorem proved...     

Nina N. Uraltseva. On the occasion of her 75<Superscript>th</Superscript> birthday

It is a great pleasure for me to write about Nina N. Uraltseva who has been one of the most supportive persons during all my scientific career. I have learned from Nina very many things starting from the foundations of Mathematics to how to prepare Swedish gravad lax (salted salmon). Nina is one of the most wholly beautiful persons I have ever met and, together with all the contributors of these anniversary issues of “Problemy Matematicheskogo Analiza,” I wish her many healthy and active years ahead. Translated from Problemy Matematicheskogo Analiza, 40, May 2009, pp. 3–6.     

“Bricks-and-mortar” vs. “clicks-and-mortar”: An equilibrium analysis

The Internet has provided traditional retailers a new means with which to serve customers. Consequently, many “bricks-and-mortar” retailers have transformed to “clicks-and-mortar” by incorporating Internet sales. Examples of companies making such a transition include Best Buy, Wal-Mart, Barnes & Noble, etc. Despite the increasing prevalence of this practice, several fundamental questions remain: (1) Does it pay off to go online? (2) Which is the equilibrium industry structure? (3) What is the implication of this business model for consumers? We study these issues in an oligopoly setting and show that clicks-and-mortar arises as the equilibrium channel structure. However, we find that this equilibrium does not necessarily imply higher profits for the firms: in some cases, rather, it emerges as a strategic necessity. Consumers are generally better off with clicks-and-mortar retailers. If firms align with pure e-tailers to reach the online market, we show that a prisoner’s dilemma-type equilibrium may arise.     

N.G. de Bruijn’s contribution to the formalization of mathematics

N.G. (Dick) de Bruijn was the first to develop a formal language suitable for the complete expression of a mathematical subject matter. His formalization does not only regard the usual mathematical expressions, but also all sorts of meta-notions such as definitions, substitutions, theorems and even complete proofs. He envisaged (and demonstrated) that a full formalization enables one to check proofs automatically by means of a computer program. He started developing his ideas about a suitable formal language for mathematics in the end of the 1960s, when computers were still in their infancy. De Bruijn was ahead of his time and much of his work only became known to a wider audience much later. In the present paper we highlight de Bruijn’s contributions to the formalization of mathematics, directed towards verification by a computer, by placing these in their time and by relating them to parallel and later developments. We aim to explain some of the more technical aspects of de Bruijn’s work to a wider audience of interested mathematicians and computer scientists.     

Crystallographic classes on the minkowski space <Emphasis Type="Italic">R</Emphasis><Subscript>1,2</Subscript>. I. Theorems

Some preliminary statements are proved necessary for calculating the crystallographic groups in six crystallographic classes in the 3-dimensional Minkowski space. Three classes are determined by the unimodular subgroups of the general Lorentz group and three more classes, by subgroups unimodular in a certain isotropic coordinate system.     

To solving multiparameter problems of algebra. 8. The <Emphasis Type="Italic">RP-q</Emphasis> method and its applications

A new method (the RP-q method) for factorizing scalar polynomials in q variables and q-parameter polynomial matrices (q ≥ 1) of full rank is suggested. Applications of the algorithm to solving systems of nonlinear algebraic equations and some spectral problems for a q-parameter polynomial matrix F (such as separation of the eigenspectrum and mixed spectrum of F, computation of bases with prescribed spectral properties of the null-space of polynomial solutions of F, and computation of the hereditary polynomials of F) are considered. Bibliography: 10 titles. __________ Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 334, 2006, pp. 149–164.     

J.-H. Lambert’s theory of probable syllogisms

In his Neues Organon of 1764, the mathematician and astronomer Jean-Henri Lambert [12] developed a theory of probable syllogisms, with the aim of formally describing the probabilist reasoning and then applying it to the probability of testimony, thus imparting an epistemic sense to the concept of probability. In the present text, we propose to study the principles of this theory and the difficulties it raises.     

Factorization of the <Emphasis Type="Italic">R</Emphasis>-matrix. II

We study the general rational solution of the Yang-Baxter equation with the supersymmetry algebra sℓ(2|1). The R-operator acting in the tensor product of two arbitrary representations of the supersymmetry algebra can be represented as a product of simpler building blocks, which are R-operators. Bibliography: 14 titles. Published in Zapiski Nauchnykh Seminarov POMI, Vol. 335, 2006, pp. 164–187.     

Factorization of the <Emphasis Type="Italic">R</Emphasis>-matrix. I

We study the general rational solution of the Yang-Baxter equation with the symmetry algebra sℓ(3). The R-operator which acts in the tensor product of two arbitrary representations of the symmetry algebra can be represented as a product of simpler “building blocks,” R-operators. The R-operators are constructed explicitly and have a simple structure. In such a way, we construct the general rational solution of the Yang-Baxter equation with the symmetry algebra s sℓ(3). To illustrate the factorization in the simplest situation, we treat also the sℓ(2) case. Bibliography: 23 titles. Published in Zapiski Nauchnykh Seminarov POMI, Vol. 335, 2006, pp. 134–163.     

On a question of B. H. Neumann

The automorphism group of a free group acts on the set of generating -tuples of a group . Higman showed that when , the union of conjugacy classes of the commutators and is an orbit invariant. We give a negative answer to a question of B.H. Neumann, as to whether there is a generalization of Higman's result for .