What do we mean when we assert that one plane figure is equal to another? In trying to find a satisfactory answer to this question I was led to consider the subjects treated in this paper.

The Ehrenfeucht Conjecture [5] states that if Μ is a finitely generated free monoid with nonempty subset S, then there is a finite subset T⊂S (a “test set”) such that given two endomorphisms f and g on Μ, f and g agree on S if and only if they agree on T. In[4], the authors prove that the above conjecture is equivalent to the following conjecture: a system of equations in a finite number of unknowns in Μ is equivalent to a finite subsystem. Since a finitely generated free monoid embeds naturally into the free group with the same number of generators, it is natural to ask whether a free group of finite rank has the above property on systems of equations. A restatement of the question motivates the following.

Let Tn be the operator algebra of upper triangular n × n complex matrices. Three families of limit algebras of the form lim (Tnk) are classified up to isometric algebra isomorphism: (i) the limit algebras arising when the embeddings Tnk→Tnk+1, are alternately of standard and refinement type; (ii) limit algebras associated with refinement embeddings with a single column twist; (iii) limit algebras determined by certain homogeneous embeddings. The last family is related to certain fractal like subsets of the unit square.

The matricial Nevanlinna–Pick interpolation criterion determines when there is an analytic matrix contraction valued function on the complex unit disc which assumes preassigned n × n matrix values w1,…,wm at preassigned interpolation points z1,…,zm. Taking ∥wi∥ < 1, for i = 1,…,m, the necessary and sufficient condition is the positivity of the nm × nm matricial Pick matrix,

The question of whether an injective module E over a noncommutative noetherian ring R remains injective after localization with respect to a denominator set X⊆R is addressed. (For a commutative noetherian ring, the answer is well-known to be positive.) Injectivity of the localization E[X-1] is obtained provided either R is fully bounded (a result of K. A. Brown) or X consists of regular normalizing elements. In general, E [X-1] need not be injective, and examples are constructed. For each positive integer n, there exists a simple noetherian domain R with Krull and global dimension n+1, a left and right denominator set X in R, and an injective right R-module E such that E[X-1 has injective dimension n; moreover, E is the injective hull of a simple module.

Let R be a (1 — 1) relation between the members of two similar classes A, B1. It correlates the members of a subclass X of A to the members of a certain subclass Y of B1 and thus defines a relation ρ connecting X and Y. It is clear that ρ is a (1 – 1) relation and that it has the property (M). If X1ρ Y1, X2ρY2, then implies

The physical observations that lead to quantitative physical theory are “pointer-readings.” The observational data consist of statements to the effect that, when one given set of pointers are incident on certain scale divisions, then another set of pointers are incident on such and such scale divisions. “Pointers” and “scale divisions” are here used in a generalised sense. The question arises as to how it is possible on the basis of a collection of incidence relations of this sort to build up a quantitative theory i.e. one involving the concept of measurement. It must be noted that until this is done any numbers associated with scale divisions serve merely as labels.

Non-Euclidean geometry in the narrowest sense is that system of geometry which is usually associated with the names of Lobachevskij and Bolyai, and which arose from the substitution for Euclid's parallel-postulate of a postulate admitting an infinity of lines through a fixed point not intersecting a given line, the two limits between the intersectors and the non-intersectors being called the parallels to the given line through the fixed point. In a wider sense, any system of geometry which denies one or more of the fundamental assumptions upon which Euclid's system is based is a non-euclidean geometry. Of special interest are, however, those which touch only the question of parallel lines ; and there exists, in addition to Lobachevskij's geometry, another, commonly associated with the name of Riemann, in which the parallels to any line through a fixed point are imaginary. The three geometries, Lobachevskij's, Euclid's, and Riemann's, thus form a trio characterised by the existence of real, coincident, or imaginary pairs of parallels through a given point to a given line. With reference to this criterion, a consistent nomenclature was introduced by Klein, who called these three geometries respectively Hyperbolic, Parabolic, and Elliptic.

Denote by f a positive measurable function on Rn, and by λ the distribution function of denotes the Lebesgue measure of the set specified. We shall suppose that λ(y)<∞ for each y>0, and that λ(y)→0 as y→∞. The decreasing rearrangement f* of f is defined on (0, ∞) by

The structure of completely prime ideals in any structural matrix near-rings is determined. Partial descriptions are obtained for prime, nil, nilpotent, and locally nilpotent ideals of structural matrix near-rings. Their associated radicals are also studied in this paper.