A palladium-catalyzed synthetic approach to new Huperzine A analogues modified at the pyridone ring

The synthesis of two new Huperzine A analogues is reported. Both products present an amino substituted benzo-fused system in place of the pyridone ring of the natural alkaloid. The synthetic strategy to the two analogues is based on three different key palladium-catalyzed steps, namely a carbonylation reaction, an epoxide isomerization and a bicycloannulation reaction.     

Down syndrome and parity

OBJECTIVE: To investigate the effect of parity on Down syndrome (DS). METHODS: The study was conducted on data from Northeast Italy (NEI) (1981-1996) and Sicily (ISMAC) (1991-1996) Congenital Malformation Registries. In these areas, all DS births are recorded and confirmed by chromosomal analysis; the NEI Registry also registers pregnancy terminations (TOPs) after prenatal diagnosis of DS. In order to estimate the effect of parity independently of the mother's age and to reduce the truncation effect, different age classes and three classes of parity (1, 2-4, >4) were defined. RESULTS: The study sample consisted of 1,088 consecutive newborns and 169 consecutive fetuses affected by DS. In both NEI and ISMAC samples, we found a significantly increased risk of having a DS child for multiparas > or =35 years of age. In the NEI sample, the inclusion of TOP data did not seem to modify this finding. In the ISMAC sample, a significantly reduced risk for primiparas was found at all ages. CONCLUSIONS: Our data confirm a higher risk of having a DS child in women with parity >4. As this effect is evident only in women > or =35 years age, its practical impact is null because these women are usually offered prenatal diagnosis in any case. However, the mechanisms involved, if this association is true, are very intriguing and the observation should stimulate scientific studies allowing a better knowledge of the nondisjunction mechanism.     

Difference Equations Approach for Multi-Server Queueing Models with Removable Servers

Methodology and Computing in Applied Probability - We consider an extended form of the MX/M/c queue with two types of server groups: Static as well as dynamic (which turn on/off in a...     

On perturbation theory at finite temperature

We show that the largest temperature-dependent contribution to the fermion self-energy of order α² in QED goes as (α²T⁴/p²)ln[(E + p)/(E ? p)], where p is the 3-momentum and E is the energy, and we speculate on the form of higher-order terms in the perturbation expansion.     

Multi-layered round robin routing for parallel servers

We study a system of several identical servers in parallel, where a routing decision must be made immediately on a job’s arrival. Jobs arrive according to a Poisson process, with their processing times following a discrete distribution with finite support. The processing time of a job is known on arrival and may be used in the routing decision. We propose a policy consisting of multi-layered round robin routing followed by shortest remaining processing time scheduling at the servers. This policy is shown to have a heavy traffic limit that is identical to one in which there is a single queue (no routing) and optimal heavy traffic scheduling. In light traffic, we show that the performance of this policy is worse than round robin routing followed by shortest remaining processing time scheduling. We also quantify the difference between round robin and multi-layered round robin routing, which in turn yields insights on the relative importance of routing versus (local) scheduling in such systems. AMS subject classifications: 68M20 · 60K25 (Work done while both authors were visitors at EURANDOM, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.)     

Maximizing throughput in queueing networks with limited flexibility

We study a queueing network where customers go through several stages of processing, with the class of a customer used to indicate the stage of processing. The customers are serviced by a set of flexible servers, i.e., a server is capable of serving more than one class of customers and the sets of classes that the servers are capable of serving may overlap. We would like to choose an assignment of servers that achieves the maximal capacity of the given queueing network, where the maximal capacity is λ^∗ if the network can be stabilized for all arrival rates λ < λ^∗ and cannot possibly be stabilized for all λ > λ^∗. We examine the situation where there is a restriction on the number of servers that are able to serve a class, and reduce the maximal capacity objective to a maximum throughput allocation problem of independent interest: the total discrete capacity constrained problem (TDCCP). We prove that solving TDCCP is in general NP-complete, but we also give exact or approximation algorithms for several important special cases and discuss the implications for building limited flexibility into a system.     

Dynamic server allocation for unstable queueing networks with flexible servers

This paper is concerned with the dynamic assignment of servers to tasks in queueing networks where demand may exceed the capacity for service. The objective is to maximize the system throughput. We use fluid limit analysis to show that several quantities of interest, namely the maximum possible throughput, the maximum throughput for a given arrival rate, the minimum arrival rate that will yield a desired feasible throughput, and the optimal allocations of servers to classes for a given arrival rate and desired throughput, can be computed by solving linear programming problems. We develop generalized round-robin policies for assigning servers to classes for a given arrival rate and desired throughput, and show that our policies achieve the desired throughput as long as this throughput is feasible for the arrival rate. We conclude with numerical examples that illustrate the points discussed and provide insights into the system behavior when the arrival rate deviates from the one the system is designed for.     

Server allocation for zero buffer tandem queues

In this paper we consider the problem of allocating servers to maximize throughput for tandem queues with no buffers. We propose an allocation method that assigns servers to stations based on the mean service times and the current number of servers assigned to each station. A number of simulations are run on different configurations to refine and verify the algorithm. The algorithm is proposed for stations with exponentially distributed service times, but where the service rate at each station may be different. We also provide some initial thoughts on the impact on the proposed allocation method of including service time distributions with different coefficients of variation.     

Exact Asymptotics for k i -limited Exponential Polling Models

In this paper we find exact asymptotic expressions for the event that the total queue length is large for a k _i -limited exponential polling model with equal service rates and two classes of customer. It is found that this behaviour divides into two very different regimes, depending on the arrival rates to the system. Using these exact asymptotic expressions, we provide heuristics for choosing the k _i values to provide a given level of quality of service to one class while giving best effort to the other class.