Developmental changes in hematopoietic stem cell properties

Hematopoietic stem cells (HSCs) comprise a rare population of cells that can regenerate and maintain lifelong blood cell production. This functionality is achieved through their ability to undergo many divisions without activating a poised, but latent, capacity for differentiation into multiple blood cell types. Throughout life, HSCs undergo sequential changes in several key properties. These affect mechanisms that regulate the self-renewal, turnover and differentiation of HSCs as well as the properties of the committed progenitors and terminally differentiated cells derived from them. Recent findings point to the Lin28b-let-7 pathway as a master regulator of many of these changes with important implications for the clinical use of HSCs for marrow rescue and gene therapy, as well as furthering our understanding of the different pathogenesis of childhood and adult-onset leukemia.     

Dines—Fourier—Motzkin quantifier elimination and an application of corresponding transfer principles over ordered fields

A constructive procedure using Dines—Fourier—Motzkin elimination is given for eliminating quantifiers in a linear first order formula over ordered fields. An ensuing transfer principle is illustrated by showing that a locally one-to-one affine map is globally one-to-one and onto all over ordered fields.This research is based on work supported in part by the National Science Foundation under Grant DMS-86-03232, by the Department of Energy grant DE-FG03-87ER25028 and by the United States-Israel Binational Science Foundation Grant 85-00295.     

A soluble-polymer system for the asymmetric transfer hydrogenation of ketones

The appropriate combination of methacrylate polymers permits the synthesis of a soluble polymer for use in ruthenium(II)-catalyzed asymmetric transfer hydrogenation reactions. Using a 7:3 copolymer of a poly(ethylene glycol) ester and a hydroxyethyl ester, a derived ruthenium(II)/norephedrine complex catalyses reduction of acetophenone in up to 95% yield and 81% ee.     

Permutation congruent transformations of the freudenthal triangulation with minimum surface density

Linear transformations of the Freudenthal triangulation with permutation congruent simplexes are characterized, and within this class those transformations yielding minimum normalized surface density are identified. This research was partially supported by National Science Foundation Grants SOC 78-16811 and MCS 81-21838; and Department of Energy Contract AM03- 76F00326, PA# DE-AT03-76ER72018.     

Magnetic-field-induced Fermi-edge singularity in the tunnelling current through a self-assembled InAs quantum dot

Tunneling transport through a one-barrier GaAs/(AlGa)As/GaAs heterostructure containing self-assembled InAs quantum dots has been investigated at low temperatures. An anomalous increase in the tunneling current through quantum dots in magnetic fields oriented both parallel and perpendicular to the current is observed. This increase is a manifestation of the Fermi-edge singularity in the current as a result of the interaction of a tunneling electron with the electron gas in the emitter.     

Magnetic-field-induced suppression of electronic conduction in a superlattice

We use a magnetic field applied along the axis of a semiconductor superlattice (SL) as a controllable means of creating a one-dimensional band structure. We demonstrate that the current flow through the SL is strongly suppressed when the electron motion perpendicular to the SL axis is strongly confined by the quantizing magnetic field. By modeling this behavior using semiclassical and nonequilibrium Green's function methods, we show that the observed quenching arises from a qualitative change in electron dynamics caused by increasing quantum confinement.     

Optimal scaling of balls and polyhedra

The concern is with solving as linear or convex quadratic programs special cases of the optimal containment and meet problems. The optimal containment or meet problem is that of finding the smallest scale of a set for which some translation contains a set or meets each element in a collection of sets, respectively. These sets are unions or intersections of cells where a cell is either a closed polyhedral convex set or a closed solid ball.This work was supported in part by the Department of Energy Contract DE-AC03-76-SF00326, PA No. DE-AT-03-76ER72018; National Science Foundation Grants MCS79-03145 and SOC78-16811; and the Army Research Office—Durham, Contract DAAG-29-78-G-0026.     

A class of “onto” multifunctions

A convex valued, upper continuous multifunction with pointed closed convex domain is proved to be onto, if there is a selection with certain coercive properties and if the selection for each point lies in the affine hull of the smallest face of the domain containing the point. The proof uses a piecewise affine homotopy construction. We apply the onto theorem to an approximation of a network of servers and show that arbitrary congestion levels can be realized with appropriate arrival levels.This research is based on work supported in part by the National Science Foundation Grant DMS 92-07409, by the Department of Energy Grant DE-FG03-87-ER-25028, by the United States—Israel Binational Science Foundation Grant 85-00295 and by ONR Grant N00014-92-J1142.     

Manipulating and imaging the shape of an electronic wave function by magnetotunneling spectroscopy

We measure the current due to electrons tunneling through the ground state of hydrogenic Si donors placed in a GaAs quantum well in the presence of a magnetic field tilted at an angle to the plane of the well. The component of B parallel to the direction of current compresses the donor wave function. By measuring the current as a function of the perpendicular component of B, we probe how the magnetocompression affects the spatial form of the wave function and observe directly the transition from Coulombic to magnetic confinement at high fields.