The inclusion of storage in a queueing system

Service operations involving an intermediate store are modelled using an extended definition of the system state which is a conventional definition of state supplemented with an indicator of the live capacity in the store. Additional to the parameters of mean service rate and mean arrival rate, a mean discharge rate of the store is introduced and the steady state probabilities are calculated in terms of these three quantities. Both finite source and infinite source cases are considered under the assumption of exponential distributions. The models permit production comparisons for systems with and for systems without storage devices.     

A Facile synthesis of 3-alkylthio-(or arylthio-)pyridines and 3,5-bis(alkylthio)pyridines

3-Alkylthio- (or 3-arylthio-)pyridines and 3,5-bis(alkylthio)pyridine can be conveniently prepared by the reaction of 3-thiocyanato- or 3,5-bis(thiocyanato)pyridines with tri-n-butylphosphine in the presence of alcohol.     

Enhancing the detection sensitivity of trace analysis of pharmaceutical genotoxic impurities by chemical derivatization and coordination ion spray-mass spectrometry

Many pharmaceutical genotoxic impurities are neutral molecules. Trace level analysis of these neutral analytes is hampered by their poor ionization efficiency in mass spectrometry (MS). Two analytical approaches including chemical derivatization and coordination ion spray-MS were developed to enhance neutral analyte detection sensitivity. The chemical derivatization approach converts analytes into highly ionizable or permanently charged derivatives, which become readily detectable by MS. The coordination ion spray-MS method, on the other hand, improves ionization by forming neutral-ion adducts with metal ions such as Na⁺, K⁺, or NH₄⁺ which are introduced into the electrospray ionization source. Both approaches have been proven to be able to enhance the detection sensitivity of neutral pharmaceuticals dramatically. This article demonstrates the successful applications of the two approaches in the analysis of four pharmaceutical genotoxic impurities identified in a single drug development program, of which two are non-volatile alkyl chlorides and the other two are epoxides.     

A mathematical model of medial consonant identification by cochlear implant users

The multidimensional phoneme identification model is applied to consonant confusion matrices obtained from 28 postlingually deafened cochlear implant users. This model predicts consonant matrices based on these subjects' ability to discriminate a set of postulated spectral, temporal, and amplitude speech cues as presented to them by their device. The model produced confusion matrices that matched many aspects of individual subjects' consonant matrices, including information transfer for the voicing, manner, and place features, despite individual differences in age at implantation, implant experience, device and stimulation strategy used, as well as overall consonant identification level. The model was able to match the general pattern of errors between consonants, but not the full complexity of all consonant errors made by each individual. The present study represents an important first step in developing a model that can be used to test specific hypotheses about the mechanisms cochlear implant users employ to understand speech.     

Bilevel programming and the separation problem

In recent years, branch-and-cut algorithms have become firmly established as the most effective method for solving generic mixed integer linear programs (MILPs). Methods for automatically generating inequalities valid for the convex hull of solutions to such MILPs are a critical element of branch-and-cut. This paper examines the nature of the so-called separation problem, which is that of generating a valid inequality violated by a given real vector, usually arising as the solution to a relaxation of the original problem. We show that the problem of generating a maximally violated valid inequality often has a natural interpretation as a bilevel program. In some cases, this bilevel program can be easily reformulated as a simple single-level mathematical program, yielding a standard mathematical programming formulation for the separation problem. In other cases, no such polynomial-size single-level reformulation exists unless the polynomial hierarchy collapses to its first level (an event considered extremely unlikely in computational complexity theory). We illustrate our insights by considering the separation problem for two well-known classes of valid inequalities.