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1.
This paper explores different means of representation for algebraic transductions, i.e., word relations realized by pushdown transducers. The relevance of this work lies more in its point of view rather than any particular result. We are aiming at giving specific techniques for obtaining, or perhaps explaining, decompositions of algebraic (and incidentally, rational) relations, relying solely on their “machine” definition rather than some complex algebraic apparatus. From this point of view, we are hoping to have demystified the heavy formalism employed in the present literature. Some of the novelties of our work are: the use of “stack languages” and “embeddings,” which eliminate the need of arbitrary context-free languages in our characterizations, the study of uniformizations for algebraic transductions and the use of the so-called stack transductions for exposing the anatomy of pushdown transducers.This work was supported by the Natural Science and Engineering Research Council of Canada grants R220259 and OGP0041630. 相似文献
2.
A.?Konstantinidis Th.?TsiatsosEmail author A.?Pomportsis 《Multimedia Tools and Applications》2009,44(2):279-304
E-learning systems have gone through a radical change from the initial text-based environments to more stimulating multimedia
systems. Such systems are Collaborative Virtual Environments, which could be used in order to support collaborative e-learning
scenarios. The main aim of this paper is to aid educational designers in selecting, designing and evaluating three dimensional
collaborative virtual environments in order to gain the pedagogical benefits of Computer Supported Collaborative Learning.
Therefore, this paper initially discusses the potential of three dimensional networked virtual environments for supporting
collaborative learning. Furthermore, based on a two-step platform selection process this paper (a) presents and compares three
dimensional multi-user virtual environments for supporting collaborative learning and (b) validates the most promising solution
against a set of design principles for educational virtual environments. According to these principles, an educational environment
has been implemented on top of the selected platform in order to support collaborative e-learning scenarios. The design of
this environment is also presented. In addition, this paper presents the results of three small scale studies carried out
in a tertiary education department, to assess the educational environment. This environment has been evaluated based on a
hybrid evaluation methodology for uncovering usability problems, collecting further requirements for additional functionality
to support collaborative virtual learning environments, and determining the appropriateness of different kinds of learning
scenarios.
相似文献
A. PomportsisEmail: |
3.
Stephanos Androutsellis-Theotokis Diomidis Spinellis Panos Louridas Kostas Stroggylos 《Computer Networks》2010,54(5):675-688
Market-based principles can be used to manage the risk of distributed peer-to-peer transactions. This is demonstrated by Ptrim, a system that builds a transaction default market on top of a main transaction processing system, within which peers offer to underwrite the transaction risk for a slight increase in the transaction cost. The insurance cost, determined through market-based mechanisms, is a way of identifying untrustworthy peers and perilous transactions. The risk of the transactions is contained, and at the same time members of the peer-to-peer network capitalise on their market knowledge by profiting as transaction insurers. We evaluated the approach through trials with the deployed Ptrim prototype, as well as composite experiments involving real online transaction data and real subjects participating in the transaction default market. We examine the efficacy of our approach both from a theoretical and an experimental perspective. Our findings suggest that the Ptrim market layer functions in an efficient manner, and is able to support the transaction processing system through the insurance offers it produces, thus acting as an effective means of reducing the risk of peer-to-peer transactions. In our conclusions we discuss how a system like Ptrim assimilates properties of real world markets, and its potential exposure and possible countermeasures to events such as those witnessed in the recent global financial turmoil. 相似文献
4.
The K-connected Deployment and Power Assignment Problem (DPAP) in WSNs aims at deciding both the sensor locations and transmit power levels, for maximizing the network coverage and lifetime objectives under K-connectivity constraints, in a single run. Recently, it is shown that the Multi-Objective Evolutionary Algorithm based on Decomposition (MOEA/D) is a strong enough tool for dealing with unconstraint real life problems (such as DPAP), emphasizing the importance of incorporating problem-specific knowledge for increasing its efficiency. In a constrained Multi-objective Optimization Problem (such as K-connected DPAP), the search space is divided into feasible and infeasible regions. Therefore, problem-specific operators are designed for MOEA/D to direct the search into optimal, feasible regions of the space. Namely, a DPAP-specific population initialization that seeds the initial solutions into promising regions, problem-specific genetic operators (i.e. M-tournament selection, adaptive crossover and mutation) for generating good, feasible solutions and a DPAP-specific Repair Heuristic (RH) that transforms an infeasible solution into a feasible one and maintains the MOEA/D’s efficiency simultaneously. Simulation results have shown the importance of each proposed operator and their interrelation, as well as the superiority of the DPAP-specific MOEA/D against the popular constrained NSGA-II in several WSN instances. 相似文献
5.
Stavros Konstantinidis 《Information and Computation》2001,167(2):120
Recently, the author introduced a nonprobabilistic mathematical model of discrete channels, the BEE channels, that involve the error-types substitution, insertion, and deletion. This paper defines an important class of BEE channels, the SID channels, which include channels that permit a bounded number of scattered errors and, possibly at the same time, a bounded burst of errors in any segment of predefined length of a message. A formal syntax is defined for generating channel expressions, and appropriate semantics is provided for interpreting a given channel expression as a communication channel (SID channel) that permits combinations of substitutions, insertions, and deletions of symbols. Our framework permits one to generalize notions such as error correction and unique decodability, and express statements of the form “The code K can correct all errors of type ξ” and “it is decidable whether the code K is uniquely decodable for the channel described by ξ”, where ξ is any SID channel expression. 相似文献
6.
George C. Papanicolaou Lykourgos C. Kontaxis Anastasia F. Koutsomitopoulou Stephanos P. Zaoutsos 《应用聚合物科学杂志》2015,132(12)
The purpose of the present study is to investigate the quasi‐static and the viscoelastic behavior of epoxy resin reinforced with starch powder. An increase in the elastic modulus on the order of 42% was achieved; a behavior that was predicted by the modulus prediction model (MPM). Next, the composite was subjected to flexural relaxation experiments, in order to determine the relaxation modulus, at different filler‐weight fractions and flexural deflections imposed. The viscoelastic models of the standard linear solid, the power law model and the residual property model (RPM) were applied in order to simulate/predict the stress relaxation curves. Predicted values derived from the application of the above models were compared to each‐other as well as to respective experimental findings. From the above comparison it was proved the superiority of the RPM model in predicting both the linear and the nonlinear viscoelastic response of the materials investigated. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41697. 相似文献
7.
Konstantinidis Andreas Thrasyvoulos Tsiatsos Theodouli Terzidou Andreas Pomportsis 《Computers & Education》2010
In this paper we examine the transferability of the Jigsaw and Fishbowl collaborative learning techniques to the Second Life platform. Our aim is to assess the applicability of Second Life for collaborative learning by developing virtual tools and metaphors and exploiting the representational richness of this novel medium. In order to enhance the existing metaphors and affordances of SL, our research team implemented educational spaces, avatar clothing, and tools for non-verbal communication and visualisation. By implementing a blended learning evaluation approach we attempted to answer three research questions focusing on student collaboration, avatar representation and learning space awareness. We can conclude that SL can supplement and/or augment face to face interactions, improving upon previous approaches in distance collaboration and communication. Furthermore, although our team augmented SL’s ability to support collaborative learning, avatar representation does not seem to scale well. Finally, the majority of the implemented affordances and metaphors seem to have enhanced collaboration and learning space awareness. 相似文献
8.
Andreas Konstantinidis Demetrios Zeinalipour-Yazti Panayiotis Andreou George Samaras Panos K. Chrysanthis 《Distributed and Parallel Databases》2013,31(2):115-149
Social communities of smartphone users have recently gained significant interest due to their wide social penetration. The applications in this domain, however, currently rely on centralized or cloud-like architectures for data sharing and searching tasks, introducing both data-disclosure and performance concerns. In this paper, we present a distributed search architecture for intelligent search of objects in a mobile social community. Our framework, coined SmartOpt, is founded on an in-situ data storage model, where captured objects remain local on smartphones and searches then take place over an intelligent multi-objective lookup structure we compute dynamically. Our MO-QRT structure optimizes several conflicting objectives, using a multi-objective evolutionary algorithm that calculates a diverse set of high quality non-dominated solutions in a single run. Then a decision-making subsystem is utilized to tune the retrieval preferences of the query user. We assess our ideas both using trace-driven experiments with mobility and social patterns derived by Microsoft’s GeoLife project, DBLP and Pics ‘n’ Trails but also using our real Android SmartP2P (http://smartp2p.cs.ucy.ac.cy/) system deployed over our SmartLab (http://smartlab.cs.ucy.ac.cy/) testbed of 40+ smartphones. Our study reveals that SmartOpt yields high query recall rates of 95 %, with one order of magnitude less time and two orders of magnitude less energy than its competitors. 相似文献
9.
Valais I.G. Michail C.M. David S.L. Konstantinidis A. Cavouras D.A. Kandarakis I.S. Panayiotakis G.S. 《IEEE transactions on nuclear science》2008,55(2):785-789
LYSO:Ce and LuYAP:Ce are single crystal non-hygroscopic scintillators of high density, high light yield and short decay time, which have been successfully used in small animal PET imagers. In the present study, the luminescence emission properties of (Lu0.9, Y0.1)2SiO5:Ce (LYSO:Ce) and (Lu0.7, Y0.3)AIO3:Ce (LuYAP:Ce) crystals were investigated for use in X-ray medical imaging. Both crystals had dimensions of 2 times 2 times 8 mm3, with all surfaces polished. Evaluation was performed by determining the X-ray luminescence efficiency (XLE) (emitted light energy flux over incident X-ray energy flux) and the detector optical gain (DOG) (emitted light photons per incident x-ray photon) in a wide range of X-ray energies employed in mammography (22-49 kVp) and in general X-ray imaging (50-140 kVp). Measurements were performed using an experimental set-up based on a photomultiplier coupled to an integration sphere. The emission spectrum under X-ray excitation was measured using an optical grating monochromator to determine the spectral compatibility to various optical photon detectors incorporated in medical imaging detectors. Optical characteristics such as transmission and absorption spectra were investigated in addition to the scintillation properties. The light emission performance of the two scintillation materials studied was found adequately high for X-ray imaging. 相似文献
10.
Lila?KariEmail author Stavros?Konstantinidis Elena?Losseva Geoff?Wozniak 《Acta Informatica》2003,40(2):119-157
An essential step of any DNA computation is encoding the input data on single or double DNA strands. Due to the biochemical properties of DNA, complementary single strands can bind to one another forming double-stranded DNA. Consequently, data-encoding DNA strands can sometimes interact in undesirable ways when used in computations. It is crucial thus to analyze properties that guard against such phenomena and study sets of sequences that ensure that no unwanted bindings occur during any computation. This paper formalizes and investigates properties of DNA languages that guarantee their robusteness during computations. After defining and investigating several types of DNA languages possessing good encoding properties, such as sticky-free and overhang-free languages, we give algorithms for deciding whether regular DNA languages are invariant under bio-operations. We also give a method for constructing DNA languages that, in addition to being invariant and sticky-free, possess error-detecting properties. Finally, we present the results of running tests that check whether several known gene languages (the set of genes of a given organism) as well as the input DNA languages used in Adlemans DNA computing experiment, have the defined properties.Received: 6 February 2003, Published online: 2 September 2003Research partially supported by Grants R2824A01 and R220259 of the Natural Sciences and Engineering Research Council of Canada. 相似文献