Randomized range-maxima in nearly-constant parallel time

Given an array ofn input numbers, therange-maxima problem is that of preprocessing the data so that queries of the type what is the maximum value in subarray [i..j] can be answered quickly using one processor. We present a randomized preprocessing algorithm that runs inO(log^* n) time with high probability, using an optimal number of processors on a CRCW PRAM; each query can be processed in constant time by one processor. We also present a randomized algorithm for a parallel comparison model. Using an optimal number of processors, the preprocessing algorithm runs inO( (n)) time with high probability; each query can be processed inO ( (n)) time by one processor. (As is standard, (n) is the inverse of Ackermann function.) A constant time query can be achieved by some slowdown in the performance of the preprocessing stage.     

VirtuE: a formal model of virtual enterprises for information markets

A vital part of a modern economy is an information market. In this market, information products are being traded in countless ways. Information is bought, modified, integrated, incorporated into other products, and then sold again. Often, the manufacturing of an information product requires the collaboration of several participants. A virtual enterprise is a community of business entities that collaborate on the manufacturing of complex products. This collaboration is often ad hoc, for a specific product only, after which the virtual enterprise may dismantle. The virtual enterprise paradigm is particularly appealing for modeling collaborations for manufacturing information products, and in this paper we present a new model, called VirtuE, for modeling such activities. VirtuE has three principal components. First, it defines a distributed infrastructure with concepts such as members, products, inventories, and production plans. Second, it defines transactions among members, to enable collaborative production of complex products. Finally, it provides means for the instrumentation of enterprises, to measure their performance and to govern their behavior.     

Inconsistent heuristics in theory and practice

In the field of heuristic search it is usually assumed that admissible heuristics are consistent, implying that consistency is a desirable attribute. The term “inconsistent heuristic” has, at times, been portrayed negatively, as something to be avoided. Part of this is historical: early research discovered that inconsistency can lead to poor performance for A^? (nodes might be re-expanded many times). However, the issue has never been fully investigated, and was not re-considered after the invention of IDA^?.This paper shows that many of the preconceived notions about inconsistent heuristics are outdated. The worst-case exponential time of inconsistent heuristics is shown to only occur on contrived graphs with edge weights that are exponential in the size of the graph. Furthermore, the paper shows that rather than being something to be avoided, inconsistent heuristics often add a diversity of heuristic values into a search which can lead to a reduction in the number of node expansions. Inconsistent heuristics are easy to create, contrary to the common perception in the AI literature. To demonstrate this, a number of methods for achieving effective inconsistent heuristics are presented.Pathmax is a way of propagating inconsistent heuristic values in the search from parent to children. This technique is generalized into bidirectional pathmax (BPMX) which propagates values from a parent to a child node, and vice versa. BPMX can be integrated into IDA^? and A^?. When inconsistent heuristics are used with BPMX, experimental results show a large reduction in the search effort required by IDA^?. Positive results are also presented for A^? searches.     

Deterministic Resource Discovery in Distributed Networks

The resource discovery problem was introduced by Harchol-Balter, Leighton, and Lewin. They developed a number of algorithms for the problem in the weakly connected directed graph model. This model is a directed logical graph that represents the vertices’ knowledge about the topology of the underlying communication network. The current paper proposes a deterministic algorithm for the problem in the same model, with improved time, message, and communication complexities. Each previous algorithm had a complexity that was higher at least in one of the measures. Specifically, previous deterministic solutions required either time linear in the diameter of the initial network, or communication complexity $O(n^3)$ (with message complexity $O(n^2)$), or message complexity $O(|E_0| \log n)$ (where $E_0$ is the arc set of the initial graph $G_0$). Compared with the main randomized algorithm of Harchol-Balter, Leighton, and Lewin, the time complexity is reduced from $O(\log^2n)$ to\pagebreak[4] $O(\log n )$, the message complexity from $O(n \log^2 n)$ to $O(n \log n )$, and the communication complexity from $O(n^2 \log^3 n)$ to $O(|E_0|\log ^2 n )$. \par Our work significantly extends the connectivity algorithm of Shiloach and Vishkin which was originally given for a parallel model of computation. Our result also confirms a conjecture of Harchol-Balter, Leighton, and Lewin, and addresses an open question due to Lipton.     

Evolution of social learning when high expected payoffs are associated with high risk of failure

In an environment where the availability of resources sought by a forager varies greatly, individual foraging is likely to be associated with a high risk of failure. Foragers that learn where the best sources of food are located are likely to develop risk aversion, causing them to avoid the patches that are in fact the best; the result is sub-optimal behaviour. Yet, foragers living in a group may not only learn by themselves, but also by observing others. Using evolutionary agent-based computer simulations of a social foraging game, we show that in an environment where the most productive resources occur with the lowest probability, socially acquired information is strongly favoured over individual experience. While social learning is usually regarded as beneficial because it filters out maladaptive behaviours, the advantage of social learning in a risky environment stems from the fact that it allows risk aversion to be circumvented and the best food source to be revisited despite repeated failures. Our results demonstrate that the consequences of individual risk aversion may be better understood within a social context and suggest one possible explanation for the strong preference for social information over individual experience often observed in both humans and animals.     

Cycle time distribution in circulating systems

The cycle time distribution (CTD) within closed, continuously circulating systems is defined and related to the residence time distributions of flow regions which make up such systems. Examples of the application of the CTD are noted and experimental methods for determining CTDs for various systems are summarized.     

Murine NIMA-related kinases are expressed in patterns suggesting distinct functions in gametogenesis and a role in the nervous system

NIMA protein kinase is a major regulator of progression into mitosis in Aspergillus nidulans. Dominant negative forms of NIMA protein prevent entrance into mitosis in HeLa cells, suggesting that mammals have a similar pathway. We have reported previously the isolation of a murine NIMA-related kinase, designated Nek1, and more recently several additional NIMA-related human kinases have been cloned. The existence of several mammalian NIMA-related genes raises the questions of whether the different mammalian members have redundant, overlapping or distinct functions, and whether these functions are related to the role of NIMA in controlling mitosis. To address these questions we have studied the expression patterns of the different murine nek genes. To this end, we isolated a murine nek2 cDNA and compared its patterns of expression, during both gametogenesis and embryogenesis, to those of nek1. Both genes were highly expressed in developing germ cells, albeit in distinct patterns. In both females and males, nek1 is expressed much earlier than nek2, suggesting only limited ability for functional redundancy. Surprisingly, a striking specificity of nek1 expression was found: high levels of nek1 RNA were observed in distinct regions of the nervous system, most notably in neurons of the peripheral ganglia. These patterns suggest that the different mammalian NIMA-related kinases participate in different phases of the meiotic process and may also have functions other than cell cycle control.     

Ring-type plasmon resonance in metallic nanoshells

A simple, approximate theoretical model of surface plasmon resonance in two-dimensional metal nanoshells is developed. The model is based on the concept of short-range surface plasmons propagating around closed circular metal nanotubes. In this model, the plasmon resonance in a metal nanotube is treated as a propagating, self-interfering plasmonic wave, in a ring-type resonance, at plasmonic wavelengths matching an integer fraction of the nanotube's effective circumference. The model is validated by detailed computer simulations based on the finite-difference time-domain method and is shown to be in full agreement with the widely used plasmon hybridization model, which is based on the quasi-static approximation.     

The limiting residence time distribution of continuous recycle systems

The limiting residence time distribution (RTD) of continuous recycle systems as the recycle ratio approaches infinity is considered. It is shown that the RTD converges to the exponential distribution whenever the system does not consist of a “dead volume” at the limit. This limiting behavior is independent of the system configuration and flow patterns. Issues concerning the proper modeling approach and the mathematical formulation of the condition that the system has no “dead volume” are discussed in detail. Some examples illustrating the importance of selecting the proper modeling approach are also provided.     

Does hormone replacement therapy inhibit coronary artery calcification?

OBJECTIVE: To determine the association between the use of hormone replacement therapy (HRT) and coronary calcium, in postmenopausal women who had no history of coronary artery disease by double helical computed tomography (CT). METHODS: We used CT to compare the prevalence and extent of coronary calcium in 41 postmenopausal women who were on HRT from the first year of menopause and 37 age-matched controls who had never used HRT. RESULTS: Both groups had a similar rate of smoking, hypertension, a positive family history, and hypercholesterolemia. Coronary calcification was observed in 28.2% of the 78 women studied. The prevalence of coronary calcium was significantly lower among HRT users: six of the 47 (14.6%), compared with 16 of the 37 nonusers (43.2%) (P < .01). The recorded risk factors had no effect on the prevalence of coronary calcium. Stepwise logistic regression analysis, including age, coronary risk factors, and HRT use as independent variables, yielded HRT as the only variable determining the presence of coronary calcium (odds ratio = 0.2; 95% confidence interval 0.06, 0.63; P = .006). CONCLUSION: The lower incidence of coronary calcium in the HRT users suggests that HRT is associated with decreased prevalence of the coronary calcification.