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We introduce a queueing system with general arrival stream and exponential service time under the N-policy, where customers may renege during idle period and arrival rates may vary according to the server's status. Probability distributions of the lengths of idle period and busy period are derived using absorbing Markov chain approach and a method to obtain the optimal control policy that minimizes long-run expected operating cost per unit time is provided. Numerical analysis is done to illustrate and characterize the method.
Economic Production Quantity (EPQ) model of a single commodity exposed to stochastic demands and product returns is considered. The production times of each product are deterministic, and times between successive demands are exponential. We also consider cancellations of orders and returns of the products which affect the level of inventory. Contrast to the traditional EPQ model, we assume that once the amount of inventory reaches to a predetermined level then the production is not halted but its speed decreases until the inventory level drops to zero. We establish probabilistic models representing those two periods, and derive the quantity of economic production.
Streptococcus mutans and Streptococcus sobrinus play important roles in dental caries. Coptis chinensis is a natural product with antimicrobial activity against enterobacteria; however, its effects on oral streptococci are still unknown. Therefore, the effects of C. chinensis on the growth and biofilm formation of the representative cariogenic bacteria S. mutans and S. sobrinus were investigated for the possible use of C. chinensis as an anticaries agent. The C. chinensis extract was diluted with sterile distilled water, and 0.1–2.5% of the extract was used in the experiment. The effects of the C. chinensis extract on the growth and glucan formation of S. mutans and S. sobrinus were measured by viable cell counting and spectrophotometry at 650 nm absorbance, respectively. Crystal violet staining was also carried out to confirm the C. chinensis extract's inhibitory effect on biofilm formation. The C. chinensis extract significantly inhibited the growth of S. mutans and S. sobrinus at concentrations of ≥ 0.3% as compared with the control group. The viable cell count of colonies decreased by 1.7-fold and 1.2-fold at 2.5% and 1.25%, respectively, compared with the control group. The biofilm formation of S. mutans and S. sobrinus was inhibited by > 20-fold at C. chinensis extract concentrations of ≥ 1.25% as compared with the control group. In summary, the C. chinensis extract inhibited the growth and biofilm and glucan formation of S. mutans and S. sobrinus . Therefore, C. chinensis might be a potential candidate for controlling dental caries.
An inventory model where the demands to the retailers are correlated with each other is considered. It is common, because of the fashion or the seasonal factors, that customers are likely to prefer buying some products at the similar moment, which may result in the correlation between the demands of retailers. In this research, we consider a twoechelon inventory model with one external supplier, one warehouse and two retailers. We derive the demand process to the warehouse and conduct experiments by simulations to characterize the effect of correlated demands on the inventory of the warehouse. It is concluded that higher inventory level is required to keep the stockout probability low when there exists positive correlation between the demands of the retailers.
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정보ㆍ통신기술의 급격한 발전으로 인해 온라인 시장은 매우 빠른 속도로 성장하고 있다. 따라서 오프라인 시장과는 다른 행태를 보이는 온라인 시장의 고객 구매 패턴과 온라인 시장의 특성에 대한 보다 면밀한 연구가 필요하다. 일반적으로 온라인 시장의 소비자는 오프라인 시장의 소비자보다 의사결정의 변화성이 심하기 때문에 온라인 시장에 참여하고 있는 기업들은 온라인 고객의 구매 패턴을 파악하고 그에 맞게 적절한 전략을 취해야 한다. 특히 재고관리의 관점에서 보면, 온라인 시장에서는 고객의 주문취소 또는 반품이 오프라인 시장보다 빈번하게 발생하므로 온라인 시장에 참여하고 있는 기업들은 합리적인 재고관리를 위해서 주문취소 및 반품을 보다 중요한 요소로 고려해야 한다. 따라서 본 연구에서는 기존 주문취소를 다루었던 연구들에서 보편적으로 사용되는 ‘주문취소를 고려하여 실제 주문의 도착률을 수정하여 사용하는 방법'과 ‘주문취소율을 하나의 파라미터로 설정하여 실제 주문과 주문취소율을 동시에 고려하는 방법'의 특성을 비교하고자 한다. 이를 위해 본 연구에서는 도착과정(수요, 주문취소, 반품)은 확률적으로 발생하는 마코프 과정(Markov process)이라고 가정하여 재고모형을 구축한 후, 연속조사와 주기조사(continuous review and periodic review) 두 경우에 대해 재고 시스템의 사이클 타임의 확률분포와 기댓값을 도출한다. The scale of on-line market is growing rapidly by radical change of information and communication technologies. So, it would be necessary to investigate behaviors of purchasing patterns of online customers as well as characteristics of on-line market. Usually, since consumers of on-line market compared to those of off-line market change their decisions frequently the companies involved in on-line market should figure out the purchasing patterns of on-line customers and then establish various strategies suitable for the current situation. In particular, from the inventory control point of view, the companies involved in on-line market must consider the order cancellation and returns of products as one of the most important factors due to frequent changes of customers' decisions for efficient inventory control. In this research, we compare the performances of two methods: (i) method which modifies the real arrival rate of order with considering order cancellations or returns and (ii) method which consider both rates of order and order cancellations or returns simultaneously without modification of the real arrival rate. For this, we investigate the inventory system with stochastic demands, returns, and cancellations. We construct an inventory model with Markovian arrival processes of demands and order cancellations(or returns) and derive the probability distribution and expectation of length of the cycle-time for both continuous review and periodic review cases. And then, we compare the expected values and variances of cycle time of the modified method and the suggested method.
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Purpose: The purpose of this study is to investigate the degrees and relationships of preterm labor stress, anxiety, and nursing needs in pregnancy. Methods: As a descriptive correlation study, data was collected from 72 pregnant women in preterm labor, admitted to 2 general hospitals and 2 women s hospitals. Data was analysed using t-test, ANOVA, pearson s correlation coefficients. Results: The mean of Preterm labor stress, anxiety, and nursing needs were 2.04±0.47, 2.31±0.59, 2.47±0.52, 2.47±0.52, respectively. The nursing needs according to the general characteristics of the subjects was found to be not significantly different. Nursing needs had a positive correlation with preterm labor stress (r=.64, p<.001) and anxiety (r=.27, p<.018). Conclusion: It is suggested to improve the satisfaction of nursing needs for women in preterm labor, it is necessary to develop and apply an educational intervention program to minimize preterm labor stress and anxiety.