A general theory on spectral properties of state-homogeneous finite-state quasi-birth-death process

Cataloged from PDF version of article.In this paper a spectral theory pertaining to Quasi-Birth±Death Processes (QBDs) is presented. The QBD, which is a generalization of the birth±death process, is a powerful tool that can be utilized in modeling many stochastic phenomena. Our theory is based on the application of a matrix polynomial method to obtain the steady-state probabilities in state-homogeneous ®nite-state QBDs. The method is based on ®nding the eigenvalue±eigenvector pairs that solve a matrix polynomial equation. Since the computational e ort in the solution procedure is independent of the cardinality of the counting set, it has an immediate advantage over other solution procedures. We present and prove di erent properties relating the quantities that arise in the solution procedure. By also compiling and formalizing the previously known properties, we present a formal uni®ed theory on the spectral properties of QBDs, which furnishes a formal framework to embody much of the previous work. This framework carries the prospect of furthering our understanding of the behavior the modeled systems manifest. Ó 2001 Elsevier Science B.V. All rights reserved

Models of production lines as quasi-birth-death processes

The aim of this work is to illustrate the suitability of quasi-birth-death processes (QBDs) for stochastic modelling of production lines. With this end in mind, first, an introduction to QBDs is made, so that the reader who may not be acquainted with this aspect of stochastic modelling may be introduced to the basics of the topic. Then, a formal definition of QBD is given and the QBDs are contrasted with the traditional birth-death processes. Later, examples of QBD models pertaining to production lines are presented. The rational of this exposition is to show how QBDs present themselves within the context of production lines and to show the kind of work that needs to be performed to fully specify the corresponding QBD. By compiling the aforementioned models, the strength of QBDs in modelling production lines is demonstrated. © 2002 Elsevier Science Ltd. All rights reserved

Variance reduction techniques: Experimental comparison and analysis for single systems

We provide a thorough analysis of the effectiveness of different Variance Reduction Techniques (VRTs). We consider both stand-alone and combined applications of two input techniques, Antithetic Variates (AV) and Latin Hypercube Sampling (LHS), and two output techniques, Control Variates (CV) and Poststratified Sampling (PS). Previous research in the area mainly focuses on asymptotic variance reduction. In this experimental study, we measure the performance of VRTs under finite simulation run lengths and analyze their effects. Our findings show that the asymptotic variance reduction results do not readily apply to finite-length simulations. We consider three different types of systems (M/M/1, serial production line and (s, S) inventory control systems) and compare the VRTs under various experimental conditions. We observe that a variance reduction cannot be guaranteed for every instance a VRT is applied. Our results also indicate that the output VRTs (CV, PS) are better than input VRTs (AV, LHS) on the average for the single systems considered in this study. More interestingly, the less-sophisticated techniques (AV, CV) often perform better than the relatively more-complex techniques (LHS, PS). A comprehensive bibliography is also provided

Ultrastructural morphology and pathogenesis in Alzheimer's disease

Biopsy specimens of the cerebral cortex from two histologically confirmed cases of Alzheimer's disease were examined by means of electron microscopy. The senile plaques consisted of a complex of abnormal dendrites, axons and extracellularly located amyloid. Alzheimer's neurofibrillary tangles were made of bundles of filaments in the pericaryon. Amyloid in the plaques and in the liver showed the same ultrastructural morphology. A suggestion is made that the changes observed at the synaptic level are possibly responsible for memory defects in the disease. No evidence was found to suggest any viral or vascular aetiology in the disease. (Journal received: 18 January 1977

Unusual association of neurofibromatosis Type 1 and idiopathic syringomyelia-bulbia-pontia

Since the association of syringomyelia and neurofibromatosis is mostly related to intramedullary tumors, syringomyelia in neurofibromatosis type 1 (NF-1) is much rarer than in neurofibromatosis type 2 (NF-2). In this paper, we present a case of NF-1 associated with an idiopathic syringomyelia extending from conus medullaris to the upper pens and discuss the relationship between neurofibromatosis and syringomyelia

PERINATAL OUTCOMES OF PREGNANT WOMEN WITH TYPE 1 DIABETES MELLITUS: COMPARISON OF MULTIDOSE INJECTION AND CONTINUOUS SUBCUTANEOUS INSULIN INFUSION

Objective. To evaluate obstetric and neonatal outcomes of patients with type 1 diabetes mellitus (T1DM) and compare multidose injection (MDI) and continuous subcutaneous insulin infusion (CSII)