Showing papers by "National University of Singapore published in 1987"

Analytical methods for the extraction of solar-cell single- and double-diode model parameters from I-V characteristics

Abstract: Analytical solutions for the rapid extraction of single- and double-diode model parameters from experimental data are described. The resulting parameters' values are shown to have less than 10 percent error for most solar cells. Error contours are also illustrated to define the range of validity of these methods.

Locking Performance in Centralized Databases

Abstract: An analytic model is used to study the performance of dynamic locking. The analysis uses only the steady-state average values of the variables. The solution to the model is given by a cubic, which has exactly one valid root for the range of parametric values that is of interest. The model's predictions agree well with simulation results for transactions that require up to twenty locks. The model separates data contention from resource contention, thus facilitating an analysis of their separate effects and their interaction. It shows that systems with a particular form of nonuniform access, or with shared locks, are equivalent to systems with uniform access and only exclusive locks.Blocking due to conflicts is found to impose an upper bound on transaction throughput; this fact leads to a rule of thumb on how much data contention should be permitted in a system. Throughput can exceed this bound if a transaction is restarted whenever it encounters a conflict, provided restart costs and resource contention are low. It can also be exceeded by making transactions predeclare their locks. Raising the multiprogramming level to increase throughput also raises the number of restarts per completion. Transactions should minimize their lock requests, because data contention is proportional to the square of the number of requests. The choice of how much data to lock at a time depends on which part of a general granularity curve the system sees.

The risk of perforation in the curved canal: anticurvature filing compared with the stepback technique.

Abstract: Summary. Anticurvature filing using the stepback technique was compared with the standard circumferential stepback method, in the mesial roots of 30 extracted human mandibular teeth. Anticurvature filing was shown to reduce the risk of perforation through the furcal surface of the root. A greater risk of perforation into the furcation was found at a level 8 mm from the apex than at 5 mm.

Transgenic Plants of Brassica napus L.

Abstract: Longitudinal stem sections of Brassica napus L. cv. Westar were co–cultivated with Agrobacterium tumefaciens A208–E, carrying the disarmed plasmid pTiT37–SE and the binary vector pMON809. The vector contains a mouse mutant dihydrofolate reductase (dhfr) coding sequences driven by the cauliflower mosaic virus 35S promoter. Inoculated explants produce transgenic shoots in selective media containing methotrexate (MTX) at a frequency of 10%. The transgenic plants are phenotypically normal, express the 35S–dhfr mRNA and are resistant to high concentrations of MTX. The mouse mutant dhfr gene is transmitted to and expressed in the seed progeny as a dominant Mendelian trait.

Perception of children's influence on family decision processes

Abstract: While several studies have examined the roles of husbands and wives in making decisions about products, few have examined the impact of children. This article reports the results of a 1985 study of the influence of children on families. The study examines children's influence in each of four stages of the purchase decision, for 25 products, and by age of the children. For child‐centered (e.g., toys, children's clothing, food) and child‐used products or services (e.g., vacations, restaurant choices, outside entertainment), the study shows that children are perceived as influential by most households. Older children are perceived as more influential than younger children for nearly all the products studied. The study concludes that “family” decision making is quite different from “husband‐wife” decision making.

A Diagnostic Test for the Multinomial Logit Model

Abstract: I propose a Lagrange multiplier test for the multinomial logit model against the dogit model (Gaudry and Dagenais 1979) as the alternative hypothesis. In view of the well-known drawback of the restrictive property of independence from irrelevant alternatives implied by the multinomial logit model, a specification test has much to recommend it. Finite sample properties of the test are studied using a Monte Carlo experiment, and the test's power against the nested multinomial logit model and the multinomial probit model is investigated. The test is found to be sensitive to the values of the regression parameters of the linear random utility function.

Spore density and root colonization of vesicular-arbuscular mycorrhizas in tropical soil

Abstract: Spore density and mycorrhizal colonization were studied in four plant species within two sites in a lowland rain forest for a period of 17 months. Although the dominant VA endophyte and plant species examined in each site were different, the variation in spore numbers and mycorrhizal colonization in both sites showed a similar pattern. When spore numbers were high, percentage colonization was low, but as spore numbers began to decline the colonization increased. Highest spore numbers were recorded from Aug. to Oct. while most mycorrhizal colonization occurred from Dec. to March. To account for this variation, the possible interaction between the nutrient status of tropical soils and host plants with the life cycle of the VA endophytes is discussed.

Axial and lateral response of pile groups embedded in nonhomogeneous soils

Abstract: A numerical method of analysis based on elasticity theory is presented for the analysis of axially and laterally loaded pile groups embedded in nonhomogeneous soils. The problem is decomposed into two systems, namely the group piles acted upon by external applied loads and pile-soil interaction forces, and a layered soil continuum acted upon by a system of pile-soil interaction forces at the imaginary positions of the piles. The group piles are discretized into discrete elements while the nonhomogeneous soil behaviour is determined from an economically viable finite element procedure. The load-deformation relationship of the pile group system is then determined by considering the equilibrium of the pile-soil interaction forces, and the compatibility of the pile and soil displacements. The influence of soil nonlinearity can be studied by limiting the soil forces at the pile-soil interface, and redistributing the "excess forces" by an "initial stress" process popular in elasto-plastic finite element analysis. The solutions from this approach are compared with some available published solutions for single piles and pile groups in homogeneous and nonhomogeneous soils. A limited number of field tests on pile groups are studied, and show that, in general, the computed response compares favourably with the field measurements. (Author/TRRL)

The optimal size of government

Abstract: In the economic analysis of the theory of government, two views of government are evident. The Pigovian view sees government as a benevolent actor striving to correct for the inadequacies and excesses of an unrestrained marketplace. The ‘Public Choice’ view of government portrays government as the tool of special interest groups as likely to generate distortions as to correct them. In this paper, a model of government that incorporates both views will be developed and then empirically tested. The model developed assumes that all expenditures by the government are inputs into the private sector production. Treating government expenditures as inputs into the production of private sector output, there is some optimal size of government that maximizes private sector output. The model incorporates a general production function for private sector output. Output is a function of private labor, private capital, and government expenditures. The Pigovian and Public Choice views of government are reflected in the assumed impact of G on the marginal productivities of L and K. The model is tested using United States data and a Cobb-Douglas production function. Estimates indicate that the 1983 level of government expenditures exceeds by 87 percent the level that would maximize private sector output. Reducing government from $ 491 billion to $ 263 billion and shifting the freed labor to the private sector would increase output from $ 1187 billion to $ 1451 billion.

Logarithmic PCM weighted QAM transmission over Gaussian and Rayleigh fading channels

Abstract: The transmission of 8-bit μ-law PCM signals using M-level weighted QAM (WQAM), M =16, 64 and 256, over Gaussian and Rayleigh fading channels is examined. The weighting process modifies the positions of the QAM constellation points so that the overall distortion in the recovered information-bearing source signal is reduced. The PCM bits are mapped to the WQAM points such that the most significant bits have a lower probability of being in error than the least significant bits. Gray coding of the constellation points is also used. The WQAM systems have been optimised for the same average signal energy per transmitted symbol as for unweighted QAM. Optimised systems have also been derived for the same peak signal energy per symbol. The theoretical and simulation results using speech indicate that 16-level and 256-level WQAM have a gain of up to 3 and 5dB, respectively, over unweighted QAM for the Gaussian channel. The gains due to weighting for 16-level QAM operating over an ideal slow Rayleigh fading channel are up to 5 dB.

Parametric appraisal of the JIT system

Abstract: Much has been written on the essential features and potential benefits of the Justin-Time production system. Using a simulation model, some salient parameters such as (a)scheduling rules, (6) the effects of pull demands level, (c) the effects of the production Kanban size and the minimum Kan ban level, and (d) the significance of the job mix are evaluated. The study shows that a common prevailing practice of assigning greater priorities to jobs with higher pull frequencies is not as efficient as a shortest process time based rule. Unlike the traditional ‘push’ method, raising the pull demand in a JIT system does not ensure a high process utilization level. Results also show that an increase in the production Kanban size and minimum output Kanban level improve the mean job tardiness but causes an escalation in the output Kanban inventory level. Contrary to common conjecture, an enlarged product mix would actually produce better process utilization and mean job tardiness.

Differences between sexes in maximum jaw opening when corrected to body size.

Abstract: According to linear interincisal measurements, women have a smaller maximum jaw opening than men. In this study the difference was 2.7%. In contrast, epidemiological surveys indicate that women have a greater mobility of joints and generally more joint laxity. Covariate analysis was used to test for the difference between sexes in mean maximum passive opening adjusted according to group mean differences in other parameters of body size. The net result was that the values became more equal between sexes. A second method, employing a geometric estimation for the angle of maximum jaw opening, showed that women had a 5.4% wider range of jaw opening than men (P less than 0.057). This method avoided the problem of considering relative body size and body factors which generally correlated poorly with maximum opening. It should be noted that an increased range of opening is a measure of hyperextensibility and does not necessarily imply laxity or instability.

A simple computational procedure for optimization problems with functional inequality constraints

Abstract: In this note, a simple computational procedure is given for solving a class of optimization problems, where an objective function is to be minimized subject to conventional inequality constraints as well as to inequality constraints of the functional type.

The chemistry and biochemistry of the sweetness of sugars.

Abstract: Publisher Summary This chapter provides an overview of the chemistry of the sweetness of sugars. Sweetness is a gustatory response evoked by most sugars and is relished by humans as well as by many other organisms. The desire for sweetness appears to be universal and undisputed. Sweetness is a quality that defies definition, but whose complexity can be appreciated merely by examining the molecular structures of those compounds that elicit the sensation. They come in all molecular shapes and sizes and they belong to seemingly unrelated classes of compounds such as aliphatic and aromatic organic compounds, amino acids, peptides and proteins, carbohydrates, complex glycosides, and even certain inorganic salts. A major advance in the evolution of taste theory came with the understanding that the primary event in the initiation of a taste response involves the interaction of a stimulant molecule with a receptor located at the taste-cell plasma-membrane. This chapter discusses the early theories associated with and fundamental structural requirements for sweetness. It also explains bitterness–sweetness relationships and discusses the sensory system and the peripheral mechanisms involved in taste perception.

A generalized model of optimal due-date assignment by linear programming

Abstract: A linear-programming model to find the optimal ‘CON due-date’ is considered for n independent jobs to be processed on a single machine. The term ‘CON due-date’ stands for constant-allowance due-date, where each job receives exactly the same due-date. The measure of performance considered is a more generalized version of similar problems studied earlier. Duality theory is used to obtain an optimal solution. Some earlier studies are shown to be special cases of the model studied in this paper. Numerical examples are presented for better understanding.