Approximation Algorithms for a k-Line Center

Given a set P of n points in ℝd and an integer k ≥ 1, let w^* denote the minimum value so that P can be covered by k congruent cylinders of radius w^*. We describe a randomized algorithm that, given P and an ε > 0, computes k cylinders of radius (1 + ε) w^* that cover P. The expected running time of the algorithm is O(n log n), with the constant of proportionality depending on k, d, and ε. We first show that there exists a small ”certificate” Q ⫅ P, whose size does not depend on n, such that for any k congruent cylinders that cover Q, an expansion of these cylinders by a factor of (1 + ε) covers P. We then use a well-known scheme based on sampling and iterated re-weighting for computing the cylinders.     

MODIP revisited: re-evaluation and refinement of an automated procedure for modeling of disulfide bonds in proteins

There have been several attempts to stabilize proteins throughthe introduction of engineered disulfide bonds. For reasonsthat are currently unclear, these have met with mixed success.Hence identification of locations where introduction of a disulfidecross-link will lead to protein stabilization is still a challengingtask. A computational procedure, MODIP, was introduced morethan a decade ago to select sites in protein structures thathave the correct geometry for disulfide formation when replacedby Cys. In this study, we re-evaluated the stereochemical criteriaused by MODIP for the selection and gradation of sites for modelingdisulfides. We introduced steric criteria to check for energeticallyunfavorable non-bonded contacts with the modeled disulfide,since these can considerably offset the stabilizing effect ofthe cross-link. The performance of the refined procedure waschecked for its ability to correctly predict naturally occurringdisulfide bonds in proteins. A set of proteins in which disulfidebonds were introduced experimentally were analyzed with respectto MODIP predictions, stability and other parameters such asaccessibility, residue depth, B-factors of the mutated sites,change in volume upon mutation and loop length enclosed by thedisulfide. The analysis suggests that in addition to properstereochemistry, stabilizing disulfides occur in regions oflow depth, relatively high mobility, have a loop length greaterthan 25 and where the disulfide typically occupies a volumeless than or equal to that of the original residues.     

Increased Toughness of Zirconium-Based Bulk Metallic Glasses Tested under Mixed Mode Conditions

The effects of mixed mode loading on the fracture behavior of Zr-based bulk metallic glasses (BMGs) (Vitreloy I and Vitreloy 106) were investigated. Mixed mode I/II and mixed mode I/III fracture conditions were tested using both notched and fatigue-precracked specimens. Fully amorphous samples exhibited tremendous increases in fracture energy with the application of mixed mode loading, while partially crystalline samples exhibited more modest increases. A comparison to the behavior of other material systems (e.g., polymers, ceramics, crystalline metals, and composites) illustrates the tremendous increase in fracture energy exhibited by these BMGs under mixed mode loading conditions.     

Polyoxometalate based soft chemical route for preparation of Pt nanorods and self-assemblies

A soft chemical route is described for the preparation of platinum nanorods and self-assemblies over photochemically reduced polyoxometalate (silicotungstate) containing composite films. Transmission electron microscopy shows that the diameters of the platinum nanorods are around 55–60 nm. The formation of platinum nanorods on solid-liquid interface reactions was explained on the basis of single site growth mechanism and diffusion limitation aggregation process.     

Stress-State Effects on the Fracture of a Zr-Ti-Ni-Cu-Be Bulk Amorphous Alloy

The effects of wide changes in stress state on the flow and fracture behavior of a single Zr-Ti-Ni-Cu-Be bulk metallic glass (BMG) of known chemistry are summarized. Both compression and tension samples were tested with superimposed hydrostatic pressures up to 1230 MPa in addition to notched tension samples tested at 0.1 MPa as well as with 490 MPa superimposed pressure. A wider range of stress states were obtained by including compression experiments conducted in pressure/shear conditions with normal stresses up to 8.7 GPa. The critical shear stress at fracture in the present BMG chemistry and sample dimensions tested is relatively unaffected by these significant changes in stress state, again indicating the normal stress/pressure dependence of flow in this particular BMG chemistry and processing conditions is vanishingly small. The present results are compared to those obtained on other metallic glasses tested under similar conditions and those that exhibit inclusion-initiated failure that demonstrate a much different dependence for similar changes in stress state.     

Modeling and simulation of dynamics of chemical reactors

The rate of the homogenous exothermic hydrolysis reaction of acetic anhydride catalyzed by sulfric acid in solvent acetic acid was estimated from nonisothermal experimental batch reactor transient temperature data. Rate equations based on three different reaction mechanisms of hydrolysis published in the literature were fitted to the experimental rate data. The experimental results on runaway and limit cycle behavior obtained with this reaction were explained by using the mechanism-based rate equations for hydrolysis in the reactor dynamic models, and good agreement was obtained between the predicted and the experimental dynamic data.     

On localized control in QoS routing

In this note, we study several issues in the design of localized quality-of-service (QoS) routing schemes that make routing decisions based on locally collected QoS state information (i.e., there is no network-wide information exchange among routers). In particular, we investigate the granularity of local QoS state information and its impact on the design of localized QoS routing schemes from a theoretical perspective. We develop two theoretical models for studying localized proportional routing: one using the link-level information and the other using path-level information. We compare the performance of these localized proportional routing models with that of a global optimal proportional model that has knowledge of the global network QoS state. We demonstrate that using only coarser-grain path-level information it is possible to obtain near-optimal proportions. We then discuss the issues involved in implementation of localized proportional routing and present some practical schemes that are simple and easy to implement     

The characterization of a proposed model GaAs/anodic oxide interface

The n-GaAs/anodic oxide interface has been characterized using capacitance-voltage (C-V) measurements on MOS capacitors and current-voltage (I-V) measurements on Schottky barriers. A simple interface state model cannot explain the observed behavior. Schottky barrier measurements made on surfaces which were previously anodized with the oxide layer subsequently removed by etching show evidence for the presence of a compensated layer in the anodized semiconductor surface region. This leads to a maximum in the conduction band energy near the surface, and based on these observations a quantitative model is formulated which can partially explain some aspects of the GaAs/anodic oxide interface, including the accumulation mode capacitance dispersion observed by many authors. Further studies were made in which the oxides were grown to difference thickness and then etched to the same thickness. Measurements on these oxides show that the C-V dispersion is related to the current density during growth and to the thickness of the oxide. There is also a correlation between the temperature during oxide growth and the interface properties. Oxides grown at 65°C show significantly more dispersion than oxides grown at 0°C. The temperature during anodization is also found to influence the anodization constant. This work was supported by a research grant (ENG-76-81334) from the National Science Foundation, Washington, DC.     

Efficient time-space mappings of nested loops onto multidimensionalsystolic arrays with a flexible buffer scheme

The task of mapping a nested loop algorithm onto a multidimensional systolic array is considered. A buffer structure for the processing elements (PEs) that allows the data tokens to arrive at the PE earlier than when they are needed is proposed. Necessary and sufficient conditions for valid mappings using this buffer structure are then given. A refinement technique for deriving efficient statement level mappings from iteration level mappings is then proposed