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71.
A simple problem concerning evaluation of programs is shown to be nonelementary recursive. The problem is the following: Given an input-free programP (i.e. all variables are initially 0) without nested loops using only instructions of the formx ← 1, x ← x + y, \(x \leftarrow x\dot - y\) ,do x... end, doesP output 0? This problem has time complexity \(2^{2^{ {\mathinner{\mkern2mu\raise1pt\hbox{.}\mkern2mu \raise4pt\hbox{.}\mkern2mu\raise7pt\hbox{.}\mkern1mu}} ^2 } } \) }cn-levels for some constantc. Other results are presented which show how the complexity of the 0-evaluation problem changes when the nonlooping instructions are varied. For example, it is shown that 0-evaluation is PSPACE-complete even for the case when the nonlooping instructions are onlyx ← x + 1,if x = 0then y ←y \(y \leftarrow y\dot - 1\) . 相似文献
72.
Let be the class of real-time nondeterministic one-counter machines whose counters make at mostone reversal. Let 1 (respectively, 2) be the subclass consisting of machines whose only nondeterministic move is in the choice of when to reverse the counter (respectively, when to start using the counter). 1 and 2 are among the simplest known classes of machines for which the universe problem has been shown undecidable. (The universe problem for a class of machines is the problem of deciding if an arbitrary machine in the class accepts all its inputs.) Here, we show that the classes of languages accepted by machines in 1 and 2 are incomparable. Moreover, the union of the language classes is properly contained in the class defined by . We also, briefly, look at the closure properties of these machines.This research was supported in part by NSF Grant MCS78-01736. 相似文献
73.
Valerio Novaresio María García-Camprubí Salvador Izquierdo Pietro Asinari Norberto Fueyo 《Computer Physics Communications》2012,183(1):125-146
The generation of direct current electricity using solid oxide fuel cells (SOFCs) involves several interplaying transport phenomena. Their simulation is crucial for the design and optimization of reliable and competitive equipment, and for the eventual market deployment of this technology. An open-source library for the computational modeling of mass-transport phenomena in SOFCs is presented in this article. It includes several multicomponent mass-transport models (i.e. Fickian, Stefan–Maxwell and Dusty Gas Model), which can be applied both within porous media and in porosity-free domains, and several diffusivity models for gases. The library has been developed for its use with OpenFOAM®, a widespread open-source code for fluid and continuum mechanics. The library can be used to model any fluid flow configuration involving multicomponent transport phenomena and it is validated in this paper against the analytical solution of one-dimensional test cases. In addition, it is applied for the simulation of a real SOFC and further validated using experimental data.Program summaryProgram title: multiSpeciesTransportModelsCatalogue identifier: AEKB_v1_0Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEKB_v1_0.htmlProgram obtainable from: CPC Program Library, Queen?s University, Belfast, N. IrelandLicensing provisions: GNU General Public LicenseNo. of lines in distributed program, including test data, etc.: 18 140No. of bytes in distributed program, including test data, etc.: 64 285Distribution format: tar.gzProgramming language:: C++Computer: Any x86 (the instructions reported in the paper consider only the 64 bit case for the sake of simplicity)Operating system: Generic Linux (the instructions reported in the paper consider only the open-source Ubuntu distribution for the sake of simplicity)Classification: 12External routines: OpenFOAM® (version 1.6-ext) (http://www.extend-project.de)Nature of problem: This software provides a library of models for the simulation of the steady state mass and momentum transport in a multi-species gas mixture, possibly in a porous medium. The software is particularly designed to be used as the mass-transport library for the modeling of solid oxide fuel cells (SOFC). When supplemented with other sub-models, such as thermal and charge-transport ones, it allows the prediction of the cell polarization curve and hence the cell performance.Solution method: Standard finite volume method (FVM) is used for solving all the conservation equations. The pressure-velocity coupling is solved using the SIMPLE algorithm (possibly adding a porous drag term if required). The mass transport can be calculated using different alternative models, namely Fick, Maxwell–Stefan or dusty gas model. The code adopts a segregated method to solve the resulting linear system of equations. The different regions of the SOFC, namely gas channels, electrodes and electrolyte, are solved independently, and coupled through boundary conditions.Restrictions: When extremely large species fluxes are considered, current implementation of the Neumann and Robin boundary conditions do not avoid negative values of molar and/or mass fractions, which finally end up with numerical instability. However this never happened in the documented runs. Eventually these boundary conditions could be reformulated to become more robust.Running time: From seconds to hours depending on the mesh size and number of species. For example, on a 64 bit machine with Intel Core Duo T8300 and 3 GBytes of RAM, the provided test run requires less than 1 second. 相似文献
74.
A new machine learning system is presented in this article. It is called INNER and induces classification rules from a set of training examples. The process followed by this system starts with the random selection of a subset of examples that are iteratively inflated in order to cover the surroundings provided that they are inhabited by examples of the same class, thus becoming rules that will be applied by means of a partial matching mechanism. The rules so obtained can be seen as clusters of examples and represent clear evidence to support explanations about their future classifications and may be used to build intelligent advisors. The whole algorithm can be seen as a set of elastic transformations of examples and rules and produces concise, accurate rule sets, as is experimentally demonstrated in the final section of the article. © 2003 Wiley Periodicals, Inc. 相似文献
75.
Mónica Serrano Julio Sahuquillo Salvador Petit Houcine Hassan José Duato 《The Journal of supercomputing》2012,59(3):1533-1551
Cluster computers represent a cost-effective alternative solution to supercomputers. In these systems, it is common to constrain
the memory address space of a given processor to the local motherboard. Constraining the system in this way is much cheaper
than using a full-fledged shared memory implementation among motherboards. However, memory usage among motherboards can be
unfairly balanced. 相似文献
76.
Oscar Oliver-Didier 《住房,理论和社会》2016,33(1):1-22
The public housing programme in Puerto Rico was highly influenced by a culturally engrained logic of colonialism that aimed to achieve a behavioural transformation of the residents of public housing based on white “criollo” norms and conducts. To this day, the process of Othering with which these bodies are understood and viewed has recurrently been reproduced and redeployed utilizing the island’s old racial/colonial hierarchies. Criminality and drug-related activities are the main discursive agents employed to stigmatize the communities that live inside public housing in Puerto Rico. It is constantly inferred that most of its residents break with the behavioural norms that the rest of the population abides by. This argument is constantly utilized to perpetuate the cultural lens that views these sectors as lazy, unruly and dangerous. This highly prejudiced misconception justified the police occupations and the fences that were erected in and around public housing premises during the 1990s. However, most of these communities rarely view themselves as victims, and it is this paper’s argument that they are constantly and collectively enacting political and physical spaces of dissent, resistance and encounter that – however momentarily – are able to overturn some of these powerful ideological and spatial constructs. 相似文献
77.
Pablo Toharia Oscar D. Robles Ricardo Suárez Jose Luis Bosque Luis Pastor 《Journal of Parallel and Distributed Computing》2012
This paper presents an analysis of a Multi-GPU Multi-CPU environment, along with the different possible hybrid combinations. The analysis has been performed for a shot boundary detection application, based on Zernike moments, although it is general enough to be applied to many different application areas. A deep study of the performance, bottlenecks and design challenges is carried out showing the validity of this approach and achieving very high frame per second rates. In this paper, Zernike calculations are carried out on GPUs, taking advantage of a packing strategy proposed to minimize host-device communication time. 相似文献
78.
Caleb A. Ellefson Oscar Marin-Flores Su Ha M. Grant Norton 《Journal of Materials Science》2012,47(5):2057-2071
Molybdenum dioxide (MoO2) is a transition metal oxide with unusual metal-like electrical conductivity and high catalytic activity toward reforming
hydrocarbons. This review covers the synthesis techniques used to fabricate MoO2 in a variety of morphologies and particle sizes. Processing from molybdenum ore and reduction from MoO3 are also covered, with emphasis on reduction mechanisms and kinetic considerations. Discussions of various solution-based
and gas phase synthesis techniques shed light on strategies to achieve various unique morphologies, which leads into a brief
discussion of nanoscale MoO2. Nanoscale MoO2 is of interest for important technological applications including catalysts for partial oxidation of hydrocarbons, solid
oxide fuel cell anodes, and high-capacity reversible lithium ion battery anodes. 相似文献
79.
Tony Salvador John W. Sherry Alvaro E. Urrutia 《Information Technology for Development》2013,19(1):77-95
About 10% of the world has access to information and communication technologies (ICTs). Telecenters and cyber cafés are one prevalent way to increase access. This paper suggests increasing access through currently existing, local businesses where people already gather and where proprietors already posses existing business relationships with suppliers and customers. This paper questions the prevailing emphasis on the “cyber'' characteristics of access, e.g., computing and internet access as is currently known, and attempts to refocus the conversation by considering computing and access in the context of the “café,” e.g., as public life in the sense of Habermas, which permits an in situ evolution of relevant access. This analysis is based on extant literature and direct ethnographic research in several public places in six countries. We offer example design perspectives based on a reflection of “third places” as inspiration for appropriate innovation in the provision of computing and communications. © 2005 Wiley Periodicals, Inc. 相似文献
80.
This review discusses the latest developments in ligand design for the Pd-catalyzed asymmetric intermolecular Mizoroki-Heck reaction, from the successful phosphine/phosphinite-nitrogen ligands to the recently reported phosphite-nitrogen ligands. The presence of a biaryl phosphite group offers several advantages as a ligand scaffold for this process. 相似文献