A local instructional theory for the guided reinvention of the group and isomorphism concepts

In this paper I describe a local instructional theory for supporting the guided reinvention of the group and isomorphism concepts. This instructional theory takes the form of a sequence of key steps as students reinvent these fundamental group theoretic concepts beginning with an investigation of geometric symmetry. I describe these steps and frame them in terms of the theory of Realistic Mathematics Education. Each step of the local instructional theory is illustrated using samples of students’ written work or discussion excerpts.     

Open subsets of projective spaces with a good quotient by an action of a reductive group

The aim of the paper is to describe all open subsets of a projective space with an action of a reductive group which admits a good quotient. As in the case of Mumford’s geometric invariant theory (which concerns projective good quotients) the problem can be reduced to the case of an action of a torus. We also show how to distinguish examples of open subsets with a good quotient coming from Mumford’s theory and give examples of open subsets with non-quasi-projective quotients.     

Lattice of normal subgroups of a group of local isometries of the boundary of a spherically homogeneous tree

We describe the structure of the lattice of normal subgroups of the group of local isometries of the boundary of a spherically homogeneous tree LIsom ∂T. It is proved that every normal subgroup of this group contains its commutant. We characterize the quotient group of the group LIsom ∂T by its commutant. Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 60, No. 10, pp. 1350–1356, October, 2008.     

Designing and scaling up an innovation in abstract algebra

In this paper, we describe the process of designing and scaling up the TAAFU group theory curriculum. This work unfolded in three overlapping stages of research and design. The initial designs emerged along with local instructional theories as the result of small-scale design experiments conducted with pairs of students. A second stage of the research and design process focused on generalizing from the initial laboratory design context to an authentic classroom setting. The third (ongoing) stage involves generalizing to instructors (mathematicians) who were not involved in the design process. We describe each of these stages, and our efforts to investigate the efficacy of the resulting curriculum, in order to provide an illustrative example of the process of scaling up an innovation.     

Rank-1 quotient divisible groups

An Abelian group is called quotient divisible if it does not contain nonzero torsion divisible subgroups but does contain a free finite-rank subgroup such that the quotient group by it is divisible. In this paper, we will describe rank-1 quotient divisible groups with the help of cocharacteristics, and we will describe the endomorphisms of these groups as well. __________ Translated from Fundamentalnaya i Prikladnaya Matematika, Vol. 13, No. 3, pp. 25–33, 2007.     

Poisson Morphisms and Reduced Affine Poisson Group Actions

We establish the concept of a quotient affine Poisson group, and study the reduced Poisson action of the quotient of an affine Poisson group G on the quotient of an affine Poisson-G-variety V. The Poisson morphisms (including equivariant cases) between Poisson affine varieties are also discussed. Received April 5, 1999, Accepted March 5, 2001     

Local Instruction Theories as Means of Support for Teachers in Reform Mathematics Education

This article focuses on a form of instructional design that is deemed fitting for reform mathematics education. Reform mathematics education requires instruction that helps students in developing their current ways of reasoning into more sophisticated ways of mathematical reasoning. This implies that there has to be ample room for teachers to adjust their instruction to the students' thinking. But, the point of departure is that if justice is to be done to the input of the students and their ideas built on, a well-founded plan is needed. Design research on an instructional sequence on addition and subtraction up to 100 is taken as an instance to elucidate how the theory for realistic mathematics education (RME) can be used to develop a local instruction theory that can function as such a plan. Instead of offering an instructional sequence that “works,” the objective of design research is to offer teachers an empirically grounded theory on how a certain set of instructional activities can work. The example of addition and subtraction up to 100 is used to clarify how a local instruction theory informs teachers about learning goals, instructional activities, student thinking and learning, and the role of tools and imagery.     

The direction of an automorphism of a totally disconnected locally compact group

We describe the asymptotic behavior of automorphisms of totally disconnected locally compact groups in terms of a set of `directions' which comes equipped with a natural pseudo-metric. The structure at infinity obtained by completing the induced metric quotient space of the set of directions recovers familiar objects such as: the set of ends of the tree for the group of inner automorphisms of the group of isometries of a regular locally finite tree; and the spherical Bruhat-Tits building for the group of inner automorphisms of the set of rational points of a semisimple group over a local field. Research supported by A.R.C. Grant DP0208137.     

Local Instruction Theories as Means of Support for Teachers in Reform Mathematics Education

This article focuses on a form of instructional design that is deemed fitting for reform mathematics education. Reform mathematics education requires instruction that helps students in developing their current ways of reasoning into more sophisticated ways of mathematical reasoning. This implies that there has to be ample room for teachers to adjust their instruction to the students' thinking. But, the point of departure is that if justice is to be done to the input of the students and their ideas built on, a well-founded plan is needed. Design research on an instructional sequence on addition and subtraction up to 100 is taken as an instance to elucidate how the theory for realistic mathematics education (RME) can be used to develop a local instruction theory that can function as such a plan. Instead of offering an instructional sequence that "works," the objective of design research is to offer teachers an empirically grounded theory on how a certain set of instructional activities can work. The example of addition and subtraction up to 100 is used to clarify how a local instruction theory informs teachers about learning goals, instructional activities, student thinking and learning, and the role of tools and imagery.     

Conjugacy classes of non-connected semisimple algebraic groups

Consider a non-connected algebraic group G = G ⋉ Γ with semisimple identity component G and a subgroup of its diagram automorphisms Γ. The identity component G acts on a fixed exterior component Gτ, id ≠ τ ∈ Γ by conjugation. In this paper we will describe the conjugacy classes and the invariant theory of this action. Let T be a τ -stable maximal torus of G and its Weyl group W. Then the quotient space Gτ//G is isomorphic to (T/(1 − τ )(T))/W^τ. Furthermore, exploiting the Jordan decomposition, the reduced fibres of this quotient map are naturally associated bundles over semisimple G-orbits. Similar to Steinberg's connected and simply connected case [22] and under additional assumptions on the fundamental group of G, a global section to this quotient map exists. The material presented here is a synopsis of the Ph.D thesis of the author, cf. [15].     

Toric Kempf-Ness sets

In the theory of algebraic group actions on affine varieties, the concept of a Kempf-Ness set is used to replace the categorical quotient by the quotient with respect to a maximal compact subgroup. Using recent achievements of “toric topology,” we show that an appropriate notion of a Kempf-Ness set exists for a class of algebraic torus actions on quasiaffine varieties (coordinate subspace arrangement complements) arising in the Batyrev-Cox “geometric invariant theory” approach to toric varieties. We proceed by studying the cohomology of these “toric” Kempf-Ness sets. In the case of projective nonsingular toric varieties the Kempf-Ness sets can be described as complete intersections of real quadrics in a complex space. Published in Russian in Trudy Matematicheskogo Instituta imeni V.A. Steklova, 2008, Vol. 263, pp. 159–172.     

On the spectral analysis of quantum field Hamiltonians

We define C^∗-algebras on a Fock space such that the Hamiltonians of quantum field models with positive mass are affiliated to them. We describe the quotient of such algebras with respect to the ideal of compact operators and deduce consequences in the spectral theory of these Hamiltonians: we compute their essential spectrum and give a systematic procedure for proving the Mourre estimate.     

Defining as a mathematical activity: A framework for characterizing progress from informal to more formal ways of reasoning

The purpose of this paper is to further the notion of defining as a mathematical activity by elaborating a framework that structures the role of defining in student progress from informal to more formal ways of reasoning. The framework is the result of a retrospective account of a significant learning experience that occurred in an undergraduate geometry course. The framework integrates the instructional design theory of Realistic Mathematics Education (RME) and distinctions between concept image and concept definition and offers other researchers and instructional designers a structured way to analyze or plan for the role of defining in students’ mathematical progress.     

Children’s unit concepts in measurement: a teaching experiment spanning grades 2 through 5

We examined ways of improving students’ unit concepts across spatial measurement situations. We report data from our teaching experiment during a six-semester longitudinal study from grade 2 through grade 5. Data include instructional task sequences designed to help children (a) integrate multiple representations of unit, (b) coordinate and group units into higher-order units, and (c) recognize the arbitrary nature of unit in comparison contexts and student’s responses to tasks. Our results suggest reflection on multiplicative relations among quantities prompted a more fully-developed unit concept. This research extends prior work addressing the growth of unit concepts in the contexts of length, area, and volume by demonstrating the viability of level-specific instructional actions as a means for promoting an informal theory of measurement.     

Gauged Floer Theory Of Toric Moment Fibers

We investigate the small area limit of the gauged Lagrangian Floer cohomology of Frauenfelder [Fr1]. The resulting cohomology theory, which we call quasimap Floer cohomology, is an obstruction to displaceability of Lagrangians in the symplectic quotient. We use the theory to reproduce the results of Fukaya–Oh–Ohta–Ono [FuOOO3,1] and Cho–Oh [CO] on non-displaceability of moment fibers of not-necessarily-Fano toric varieties and extend their results to toric orbifolds, without using virtual fundamental chains. Finally, we describe a conjectural relationship with Floer cohomology in the quotient.     

On the uniqueness of roots in virtually nilpotent groups

After revisiting the concept of the torsion subgroup of a group with respect to a set of prime numbers P (as introduced by Ribenboim), we show that, for all p in P, p-th roots are unique in a virtually nilpotent group if and only if P-roots are unique in both its Fitting subgroup and its Fitting quotient. This more general notion of torsion also turns out to be sufficient to understand completely the kernel of the P-localization homomorphism of a virtually nilpotent group. Using this result, we characterize the finitely generated virtually nilpotent groups such that, when dividing out the P-torsion subgroup, P-roots exist and are unique in the resulting quotient. Received March 10, 1998; in final form July 10, 1998     

Peer Interaction in Science Concept Development and Problem Solving

Cooperative learning is commonly advocated as an effective instructional strategy in classrooms. Years of research support this recommendation. Recently, however, cognitive researchers and theorists suggest that peer group work may possibly enhance concept development and problem solving. The effectiveness of group work, including peer tutoring, cooperative learning, and peer collaboration, may be explained using several theoretical perspectives. Piaget theorized that the importance of peers comes from their ability to share ideas and initiate the equilibration process in individuals. Vygotsky argued that learning takes place in social contexts only to be internalized at a later time. He proposed a “zone of proximal development” to describe the difference between a student's ability to solve a problem alone and with the help of a more knowledgeable person. Researchers focusing on both theoretical positions argue that results support both theories. Additionally, researchers suggest that peer collaboration may enhance concept development and problem solving ability. Recommendations are made for incorporating effective peer learning strategies into instruction.     

Picard-Lefschetz theory for the coadjoint quotient of a semisimple Lie algebra

The paper develops a Picard-Lefschetz theory for the coadjoint quotient of a semisimple Lie algebra and analyzes the resulting monodromy representation of the Weyl group.Oblatum 9-IX-1993 & 15-IV-1995The author is supported by a grant from NSERC Canada.