Replenishment Period Research Articles

A Genetic Algorithmic Approach on a Deterministic Inventory Model for Deteriorating items with Shortages

The purpose of this research is to determine an optimal solution of a deterministic inventory model of single deteriorating items with a constant rate of deterioration. In this model, the demand rate is a ramp type function of time. Shortages are allowed and partially backlogged. During the shortage period, the backlogging rate is a variable which depends on the length of the waiting time over the replenishment period. The mathematical formulation of the problem indicates that the model is a non-linear constrained optimization problem. Considering the complexity of solving such a model (for getting global optima, not local optima, as it is a decision making problem.), a real-coded genetic algorithms (GAs) with Random Stochastic Sampling selection {with replacement), whole arithmetic crossover and mutation has been developed. In the algorithm, mutation is c;uTied out for the tine tuning capabilities of the system by non-uniform operators whose action depends on the age of the population. The proposed model has been solved using this real-coded GA as well as Generalised Reduced Gradient (GRG) Method. Finally, the results are illustrated with the help of a numerical example and sensitivity analysis of the optimal solution with respect to the different parameters of the system is carried out.

Assessing Immigrant Assimilation: New Empirical and Theoretical Challenges

This review examines research on the assimilation of immigrant groups. We review research on four primary benchmarks of assimilation: socioeconomic status, spatial concentration, language assimilation, and intermarriage. The existing literature shows that today's immigrants are largely assimilating into American society along each of these dimensions. This review also considers directions for future research on the assimilation of immigrant groups in new southern and midwestern gateways and how sociologists measure immigrant assimilation. We document the changing geography of immigrant settlement and review the emerging body of research in this area. We argue that examining immigrant assimilation in these new immigrant gateways is crucial for the development of theories about immigrant assimilation. We also argue that we are likely to see a protracted period of immigrant replenishment that may change the nature of assimilation. Studying this change requires sociologists to use both birth cohort and generation as temporal markers of assimilation.

THE EFFECT OF CREDIT PERIOD ON THE OPTIMAL LOT SIZE FOR DETERIORATING ITEMS WITH TIME VARYING DEMAND AND DETERIORATION RATES

In this paper, we present an inventory model for deteriorating items with time varying demand and deterioration rates when the credit period depends on the retailer's ordering quantity. We also provide simple solution procedures for finding the optimal replenishment period and show in a rigorous way that the policy suggested is indeed optimal. Further, we use numerical examples to illustrate the model and conclude the paper with suggestions for possible future research.

Analytical and experimental investigation of coolant velocity in high speed grinding

Use of water-base coolant is a pre-requisite in an high speed grinding process to avoid thermal damage and to achieve better surface integrity as well as higher grinding ratio. However, the presence of hazardous chemical additives in the coolant causes environmental problems. As a result, stringent government legislation is being practiced for the coolant use and disposal, which consumes 7–17% of the total machining cost. This paper reports the coolant flux minimization through controlled jet impingement so as to prolong the coolant replenishment cycle. Control of coolant flux was achieved through development of a “metered quantity coolant” (MQC) nozzle which supplies the required amount of coolant to the grinding zone. Also, this investigation has shown that coolant velocity has a significant influence on the high speed grinding performance. When the coolant velocity is inadequate, coolant could not penetrate into the grinding zone. The increase in coolant velocity was realized with reduction in nozzle opening area and does not use a large quantity of coolant. This is of significance to reduce environmental pollution and machining costs through extended coolant replenishment period.

Analytical and experimental investigation of coolant velocity in high speed grinding

Use of water-base coolant is a pre-requisite in an high speed grinding process to avoid thermal damage and to achieve better surface integrity as well as higher grinding ratio. However, the presence of hazardous chemical additives in the coolant causes environmental problems. As a result, stringent government legislation is being practiced for the coolant use and disposal, which consumes 7–17% of the total machining cost. This paper reports the coolant flux minimization through controlled jet impingement so as to prolong the coolant replenishment cycle. Control of coolant flux was achieved through development of a “metered quantity coolant” (MQC) nozzle which supplies the required amount of coolant to the grinding zone. Also, this investigation has shown that coolant velocity has a significant influence on the high speed grinding performance. When the coolant velocity is inadequate, coolant could not penetrate into the grinding zone. The increase in coolant velocity was realized with reduction in nozzle opening area and does not use a large quantity of coolant. This is of significance to reduce environmental pollution and machining costs through extended coolant replenishment period.

Hydrogeology as a climate recorder: Sahara–Sahel (North Africa) and the Po Plain (Northern Italy)

Information on palaeoclimate is encoded in groundwater in a variety of measurable “tracers”. A great variety of geochemical and isotopic methods has been developed and tested with the aim of defining the different moments and aspects of the water cycle, providing a fundamental information on the recharge and groundwater flow conditions. Furthermore, environmental tracers allow us to reconstruct past environmental changes. Particular emphasis is put on Western Africa and on the Po valley. In fact, two features are common to these regions: the movement of air masses crossing them and the geological characteristics. From the global perspective, Sahel, North Africa, Mediterranean Basin and Central Europe reveal a relative simple pattern of spatial and temporal composition of precipitation. The displacement of the Intertropical Convergence Zone (ITCZ) and the Azores anticyclone control the climatological regime and, consequently, the isotopic composition of rainfall. Moreover, both regions are characterised by big aquifers in the form of multilayer systems several hundred metres thick in large sedimentary basins. The aquifers are generally formed by sandstones (Sahara) or sands (Po valley) and are interbedded with argillaceous layers and outcrop only in fairly limited areas, constituting the recharge zones. The groundwater movement in confined aquifers is always lower than 1 m year −1, respecting the conditions of steady-state flow regime and hydraulic continuity, necessary for the “hydro-archives”. The comparison between the Sahara–Sahel and southern Alps allows to define the principal periods of aquifer replenishment associated with the displacement of the Intertropical Convergence Zone (ITCZ) and the Azores anticyclone. Continental isotope archives provide a means of extending the knowledge on the magnitude and the causes of environmental and climate changes. Nevertheless, for a quantitative interpretation of the isotope records preserved in continental archives, it is necessary to know the response of the isotopic composition of precipitation to long-term fluctuations in key climatic parameters over a given area. Further, the transfer functions relating the climate-induced changes of the isotopic composition of precipitation to the isotope record preserved in the given archive should be established. It appears, thus, that stable isotopes can, presently only be used as an indirect proxy dating method for palaeowaters.

The stochastic dynamic production/inventory lot-sizing problem with service-level constraints

This paper addresses the multi-period single-item inventory lot-sizing problem with stochastic demands under the “static–dynamic uncertainty” strategy of Bookbinder and Tan (Manage. Sci. 34 (1988) 1096). In the static-dynamic uncertainty strategy, the replenishment periods are fixed at the beginning of the planning horizon, but the actual orders are determined only at those replenishment periods and will depend upon the demand that is realised. Their solution heuristic was a two-stage process of firstly fixing the replenishment periods and then secondly determining what adjustments should be made to the planned orders as demand was realised. We present a mixed integer programming formulation that determines both in a single step giving the optimal solution for the “static–dynamic uncertainty” strategy. The total expected inventory holding, ordering and direct item costs during the planning horizon are minimised under the constraint that the probability that the closing inventory in each time period will not be negative is set to at least a certain value. This formulation includes the effect of a unit variable purchase/production cost, which was excluded by the two-stage Bookbinder–Tan heuristic. An evaluation of the accuracy of the heuristic against the optimal solution for the case of a zero unit purchase/production cost is made for a wide variety of demand patterns, coefficients of demand variability and relative holding cost to ordering cost ratios. The practical constraint of non-negative orders and the existence of the unit variable cost mean that the replenishment cycles cannot be treated independently and so the problem cannot be solved as a stochastic form of the Wagner–Whitin problem, applying the shortest route algorithm.

Ischemia-Reperfusion Injury of Retinal Endothelium by Cyclooxygenase- and Xanthine Oxidase-Derived Superoxide

The formation of reactive oxygen species (ROS) may be important in the pathogenesis of microvascular dysfunction and injury in ischemic retinopathies. The authors hypothesized that retinal endothelial cells can generate injurious levels of superoxide radical in response to ischemia/reperfusion, that endothelial xanthine oxidase and cyclooxygenase are important enzymatic sources of superoxide radical under these conditions, and that superoxide scavengers and inhibitors of these enzymes can protect endothelium from ischemic injury. The authors used confluent cultures of mouse retinal endothelial cells (MREC) subjected to exogenously generated superoxide or simulated ischemia-reperfusion to test these hypotheses. Cell injury was assessed biochemically by lactate dehydrogenase release into the culture medium. MREC were injured in a duration-dependent fashion by exposure to the superoxide-generating mix of hypoxanthine and xanthine oxidase. Increasing periods of oxygen and glucose deprivation (OGD) for 5–9hr followed by replenishment of substrates for 2hr led to progressive increases in endothelial cell injury; a significant proportion of the injury occurred during the period of substrate replenishment. Significant MREC protection was achieved by the superoxide scavengers SOD (1000Uml−1) and a carboxylic acid derivative of carboxyfullerene (10μM), the xanthine oxidase inhibitors oxypurinol (100μM) and diphenyleneiodonium (DPI) (100n M), and the cyclooxygenase inhibitors indomethacin (300μM) and ibuprofen (300μM). It is concluded that MREC are vulnerable to auto-oxidative injury by superoxide radical generated following a period of OGD. Both xanthine oxidase- and cyclooxygenase-dependent pathways are important enzymatic sources of superoxide formation in this setting. These enzymes and the ROS produced from their activity may be viable therapeutic targets to reduce microvascular dysfunction and injury in ischemic retinopathies.

Coordinating supply chain inventories through common replenishment epochs

This paper proposes a model to study and analyze the benefit of coordinating supply chain inventories through the use of common replenishment epochs or time periods. A one-vendor, multi-buyer supply chain for a single product is analyzed. Under the proposed strategy, the vendor specifies common replenishment periods and requires all buyers to replenish only at those time periods. The vendor offers a price discount to entice the buyers to accept this strategy. The optimal replenishment period and the price discount to be offered by the vendor are determined as a solution to a Stackelberg game. After developing a method to solve the game, a numerical study is conducted to evaluate the benefit of the proposed coordinated strategy.

An EOQ model with deteriorating items in response to a temporary sale price

In economic order quantity models, it is often assumed that the unit purchase cost is constant. Such an assumption is usually not fulfilled in many practical situations. In practice, it is observed that suppliers sometimes offer temporary price discounts to stimulate demand, boost market share or decrease inventories of certain items. In this paper, a deteriorating inventory model with a temporary sale price has been developed. We shall be concerned with finding the optimal total cost saving for deteriorating items during the special replenishment period. Numerical examples are presented to illustrate the proposed model.

Lack of Reproduction in Muskoxen and Arctic Hares Caused by Early Winter?

A lack of young muskoxen (Ovibos moschatus) and arctic hares (Lepus arcticus) in the Eureka area of Ellesmere Island, Northwest Territories (now Nunavut), Canada, was observed during summer 1998, in contrast to most other years since 1986. Evidence of malnourished muskoxen was also found. Early winter weather and a consequent 50% reduction of the 1997 summer replenishment period appeared to be the most likely cause, giving rise to a new hypothesis about conditions that might cause adverse demographic effects in arctic herbivores.Key words: arctic hare, Lepus arcticus, Ellesmere Island, malnutrition, muskox, Ovibos moschatus, population, reproduction, snow, weather, winter severity

Comparative economic analysis between direct and indirect wiring in the copper-based local loop

In a copper-based local loop, a circuit pair is indirectly connected to each subscriber usually via a primary cross connection point (PCP) which acts as a buffer to absorb circuit demand fluctuations among subareas. But some telephone operators adhere to the classic practice of direct wiring based on technological preferences without taking advantage of cost-efficient flexibility points. We analyse the extra cost of maintaining the old practice of direct wiring over the popular one using flexibility points in a single PCP area. For that, the expectation of circuit shortages in subareas during a single replenishment period for the direct wiring is first obtained. Exploiting the convexity of the expectation, we then present a procedure for optimally allocating circuits among subareas, which not only serves its own purpose of circuit provisioning for the direct wiring but also precisely calculates the extra cost over the indirect case.

Fruit Production and Seed Predation in Two Miombo Woodland Trees in Zambia

ABSTRACTDuring a six year period (1990–1995) fruiting frequency among marked trees and seed production and predispersal predation by beetle larvae of lsoberlinia angolensis and Julbernardia globiflora of the Leguminosae family and Caesalpinoideae subfamily were studied at two dry miombo woodland sites in central Zambia. Fruit production varied from year to year and was not related to annual rainfall. On average I. angolensis trees fruited once every two years while J. globiflora trees fruited once every two and one‐half years. Number of fruits per tree was correlated with tree size for both species. In a peak year fruit biomass represented two to four percent of total above ground biomass but for J. globiflora about 5.5 and 4.5 percent of nitrogen and potassium, respectively, were in fruits compared to 0.6 and 1.8 percent for I. angolensis. The lower fruiting frequency in J. globifora was probably related to the depletion of N reserves during a fruiting year which required a longer replenishment period than in I. angolensis.Predispersal seed predation was low for J. globiflora (10%) and very high in I. angolensis (65–85%) and for the latter species predation rate increased from August to November 1991 and significant variations were observed among years and pod types. Seed predation rate was lowest in a peak fruiting year and this observation provides support for the satiation hypothesis. The results of the study also suggest that I. angolensis and J. globiflora may have different regeneration strategies although reproductive allocation in both species was within the range reported for other iteroparous plants.

An EOQ Model for Deteriorating Items with Time Varying Demand and Costs

A generalized EOQ model for deteriorating items is considered here in which the demand rate, deterioration rate, holding cost and ordering cost are all assumed to be continuous functions of time. Shortages are also allowed and are completely backlogged. The planning horizon is finite and the replenishment periods are assumed to be constant. The optimal replenishment policy and the decision rule which minimizes the total system cost are derived. A numerical example is given to illustrate the developed model. Sensitivity analysis is also presented for the given model.

An EOQ Model for Deteriorating Items with Time Varying Demand and Costs

A generalized EOQ model for deteriorating items is considered here in which the demand rate, deterioration rate, holding cost and ordering cost are all assumed to be continuous functions of time. Shortages are also allowed and are completely backlogged. The planning horizon is finite and the replenishment periods are assumed to be constant. The optimal replenishment policy and the decision rule which minimizes the total system cost are derived. A numerical example is given to illustrate the developed model. Sensitivity analysis is also presented for the given model.

Bioavaliability of Fe, Ca, P and Mg in Fe-deficient rats treated with different sources of dietary iron

Iron deficiency (D0) markedly decreases the concentration of iron and calcium in the liver, femur and sternum. We investigated the effect of the source of dietary iron [ferric citrate alone (citrate diet), or combined with bovine blood at a proportion of 1:1 (citrate + heme diet), and ferric protein (protein-Fe diet)] on the nutritive utilization of Fe, Ca, P and Mg in anemic and healthy rats. All diets contained the same amount of Fe (40 mg/kg diet). In Fe-deficient rats (DT) fed citrate, citrate + heme or protein-Fe diet the apparent digestibility coefficient (ADC) of Fe were 39%, 37% and 94% greater than in the respective control groups (C); hemoglobin regeneration efficiencies (HRE) were respectively 47%, 51%, and 172% greater than in the control groups. The ADC of Ca was not affected by Fe deficiency, whereas the ADC of Mg decreased significantly, and that of P increased under our experimental conditions. In Fe-deficient rats fed Fe for 10 days (DT), the concentrations of hemoglobin approached normal values. However, serum Fe remained low, indicating that Fe reserves were still depleted. Hepatic and femoral Fe concentrations were also lower in all DT groups, whereas Fe concentrations in the sternum increased significantly, suggesting an increase in erythropoiesis. After supply of dietary Fe for 10 days in iron-deficient groups (DT), Ca and P concentrations increased in sternum, a site assumed to have higher requirements for these minerals. In the liver these values tended to normalize, whereas in the femur Ca and P stores remained depleted. Mg concentrations after the replenishment period generally approached normality in all three organs.

Spatial and seasonal variation of methane emission from an Amazon floodplain lake

Methane (CH4) emission from a floodplain lake in central Amazonia was measured during a complete annual cycle using a static chamber method. The results show significant differences in the CH4 flux rates from different habitats in the lake: open water, floating vegetation (mainly the aquatic grass Paspalum repens) and floodplain forest. The highest CH4 emission rates (10–198 mg CH4 m−2 d−1) were recorded during inundation in the seasonally flooded forest. Lower emission rates occurred in the floating vegetation (3–79 mg CH4 m−2 d−1), which is found in areas with a short subaerial phase and in the permanently aquatic part of the lake (4–65 mg CH4 m−2 d−1) free of macrophytes and with the lowest productivity. Seasonal fluctuations in water level resulted in distinct seasonal patterns in CH4 release at different sites in the lake. The permanently aquatic part of the lake had lowest emission rates during periods of water inflow and subsurface oxygen replenishment into the lake. At the periodic aquatic...

Period and phase of customer replenishment: A new approach to the Strategic Inventory/Routing problem

The delivery of inventory to customers involves the use of vehicles for which purchase or lease agreements may need to be signed months or even years before the start of actual delivery operations. Such long lead times make it impractical to build a fleet incrementally and motivated study of the Strategic Inventory Routing Problem (SIRP). SIRP focuses on estimating, in advance of the start of actual delivery operations, the minimum size (or cost) vehicle fleet required to supply inventory from a central depot to spatially dispersed customers. A fundamental difference between SIRP and the associated tactical IRP's (route an existing fleet, given knowledge of actual customer inventory levels) is that all possible realizations of the tactical problem must be considered, at least implicitly, when solving SIRP. This paper generalizes Larson's approach for SIRP through the use of period and phase of customer replenishment as additional decision variables. Routing solutions based on customer-specific period and phase of replenishment are developed for a simple model of the tactical routing problems the fleet will eventually encounter. Estimates of the fleet size required are developed on the basis of these routing solutions. The period/phase approach can be generalized to take long-term operating costs into account. Computational tests show that the new approach yields significant reductions in solution cost when the vehicle is large enough to replenish several customers in a single trip and/or when there is significant variation in the maximum inter-replenishment intervals of the customers.

NUTRITIONAL ASPECTS OF HIV INFECTION

Wasting may not be an inevitable consequence of HIV infection but may be a consequence of multiple nutritional insults that are additive without periods of replenishment in between. Protein energy malnutrition in AIDS patients may be consequential to underlying illness and concomitant to death as a result of that illness or may hasten a patient's demise, that is, starvation with fatal loss of body cell mass. Mortality is closely related to weight loss. Malnutrition may be a result of decreased intake, malabsorption, altered metabolism, or any combination of the three. Nutritional strategies to prevent PEM include appetite stimulation, early nutritional supplementation with oral supplements, and the diagnosis and treatment of malabsorption and underlying infections. More aggressive measures such as gastrostomy or jejunostomy tube placement and total parenteral feedings are still being evaluated. Nutritional supplementation to enhance the immune system or manipulate metabolism may be adjunctive to the above strategies. Early intervention and attention to nutritional status may have long-term benefits to patients with this disease.

Lot‐sizing Time‐varying Demands: A Spreadsheet Approach

Describes a spreadsheet approach for implementing Wagner‐Whitin (WW) and Silver‐Meal (SM) methods for lotsizing time‐varying demands. Suggests that this approach manages the computation and makes tedious and repetitive tasks more interesting; consequently, the student/user can concentrate on the concepts. Outlines the methods, which involve the computation of incremental costs (inventory carrying and replenishment costs) for several alternatives in each period. Presents a spreadsheet template to compute the incremental costs for possible alternatives which is created by using only a small number of spreadsheet formulae and a series of copy commands. The costs are displayed as a matrix. Describes a step‐by‐step procedure to determine the period and the quantity of replenishment.