169,940 research outputs found
Efficient Identification of Timed Automata: Theory and practice
No full textThis thesis contains a study in a subfield of artificial intelligence, learning theory, machine learning, and statistics, known as system (or language) identification. System identification is concerned with constructing (mathematical) models from observations. Such a model is an intuitive description of a complex system. One of the main nice properties of models is that they can be visualized and inspected in order to provide insight into the different behaviors of a system. In addition, they can be used to perform different calculations, such as making predictions, analyzing properties, diagnosing errors, performing simulations, and many more. Models are therefore extremely useful tools for understanding, interpreting, and modifying different kinds of systems. Unfortunately, it can be very difficult to construct a model by hand. This thesis investigates the difficulty of automatically identifying models from observations. Observations of some process and its environment are given. These observations form sequences of events. Using system identification, we try to discover the logical structure underlying these event sequences. A well-known model of such a logical structure is the deterministic finite state automaton (DFA). A DFA is a language model. Hence, its identification (or inference) problem has been well studied in the grammatical inference field. Knowing this, we want to take an established method to learn a DFA and apply it to our event sequences. However, when observing a system there often is more information than just the sequence of symbols (events): the time at which these symbols occur is also available. A DFA can be used to model this time information implicitly. A disadvantage of such an approach is that it can result in an exponential blowup of both the input data and the resulting size of the model. In this thesis, we propose a different method that uses the time information directly in order to produce a timed model. We use a well-known DFA variant that includes the notion of time, called the timed automaton (TA). TAs are commonly used to model and reason about real-time systems. A TA models the timed information explicitly, i.e., using numbers. Because numbers use a binary representation of time, such an explicit representation can result in exponentially more compact models than an implicit representation. Therefore, also the time, space, and data required to identify TAs can be exponentially smaller than the time, space, and data required to identify DFAs. This efficiency argument is our main reason we are interested in identifying TAs. The work in this thesis makes four major contributions to the state-of-the-art on this topic: 1. It contains a thorough theoretical study of the complexity of identifying TAs from data. 2. It provides an algorithm for identifying a simple TA from labeled data, i.e., from event sequences for which it is known to which type of system behavior they belong. 3. It extends this algorithm to the setting of unlabeled data, i.e., from event sequences with unknown behaviors. 4. It shows how to apply this algorithm to the problem of identifying a real-time monitoring system. These contributions are of importance for anyone who is interested in identifying timed systems. Most importantly, both in our theoretical work and in our experiments we show that identifying a TA by using the time information directly is more efficient than identifying an equivalent DFA. In addition, our techniques can be applied to many interesting problems due to their generality. Examples are gaining insight into a real-time process, recognizing different process behaviors, identifying process models, and analyzing black-box systems.Software TechnologyElectrical Engineering, Mathematics and Computer Scienc
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Mitomycin C in highly myopic eyes - Author reply
No full textOphthalmology. 2005 Feb;112(2):208-18; discussion 219.
Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes.
Gambato C, Ghirlando A, Moretto E, Busato F, Midena E.
SourceRefractive Surgery Service and Antimetabolite Therapy Research Unit, Department of Ophthalmology, University of Padova, Padova, Italy.
Abstract
PURPOSE: To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.
DESIGN: Prospective, double-masked, randomized clinical trial.
PARTICIPANTS: Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.
METHODS: In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).
MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.
RESULTS: Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).
CONCLUSION: Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.
Comment in
Ophthalmology. 2006 Feb;113(2):357; author reply 357-8
Detecting malicious behaviour using system calls
No full textThe emergence of Apple’s Macintosh computers’ popularity introduces new threats and challenges for the security on the Mac. For a long time, OS X security has benefitted from the popularity of Microsoft Windows. The threat landscape for the Mac is rapidly changing as the marketshare of the Mac is approaching 15%1. Malware on Apple’s OS X systems emerges to be an increasing security threat that is currently solely countered with ancient anti-virus (AV) technologies [18]. Current AV technologies pose a performance overhead on the entire system and have an inherent delayed effectiveness, due to their signature based detection [15][31]. In addition, current malware uses many forms of obfuscation to prevent detection by AV technologies, redering AV technologies useless against advanced threats [15][31]. Consequently, the need for more advanced detection and prevention techniques of malware is increasing. Detection of malicious behaviour instead of malicious signatures, ought to provide a more advanced form of protection. A system call is referred to as the request and service of specific, basic, functionality provided to applications by the operating system. This Master thesis answers the research question: “Is it possible to detect malicious behaviour per- formed by malware, based on monitoring system calls?” Presented is a novel, generic, behavioural detection and prevention mechanism for malware on OS X based on system calls. System call traces can be used to describe the behaviour of processes [11]. Much effort was put into the development of a kernel module that bypasses kernel security mechanisms and rewires one of the operating system’s core functionalities; system call handling. The rewiring of system call handling provided the ability to log all of the system call invocations performed by processes running on the monitored system. A significant amount of OS X malware and benign applications were executed in a monitored environment of which system call traces were collected. Based on analysing heat map visualisations and manual sequential analysis of the system call traces of both malicious and benign processes, anomalies in the malicious traces could be observed. Subsequently, several mali- cious system call patterns and detection rules were extracted providing detection of malware on OS X. The most successful defined pattern is constructed around the executions of Unix shell processes per- formed by malware. It is shown that this detection pattern results in a 100% detection rate of all malware possible to obtain for this thesis. Even advanced malware in an infected OS X application, known as OSX.KeyRanger.A, was detected using this method. In order to evaluate the False Positive Rate (FPR) accurately in real world scenarios, three different user profiles were defined. Applications distributed via the Mac App Store do not generate false positives. In case of the developer user profile type, the FPR increases to 20%. Applications responsible for the false positives feature a cross-platform nature, such as MATLAB, R, LaTeX and interpreters for scripting languages. A conducted survey under Mac users verified these conclusions. However, the number of false positive generating benign applications is very limited and whitelisting solutions provided can reduce the FPR in this developer user profile.Electrical Engineering, Mathematics and Computer ScienceIntelligent System
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
A Multi-Language Comparison of Influences on Author Verification using Character N-Grams
No full textWe create a new multi-language corpus for author verification based on Wikipedia talkpages, and evaluate the influence that differences in topic and time have on character n-gram author profiles. Topic alignment between two texts is found to increase author verification precision, and an authors writing style is found to change over time, but not more significantly after 3 years than after 1 year.Information ArchitectureWISElectrical Engineering, Mathematics and Computer Scienc
Context-Based Spelling Correction for the Dutch Language: Applied on spelling errors extracted from the Dutch Wikipedia revision history
No full textIn this thesis we did research on context-based spellchecking approaches for the Dutch language. Context-based approaches enable the detection of real-word spelling errors by using the context in which the errors occur. We also assessed if we could improve the ranking of replacement candidates by using the context. To be able to measure the performance of the different techniques used, a dataset containing erroneous-corrected sentence pairs was obtained from the Dutch Wikipedia revision history. This dataset contains a wide variety of human generated spelling errors, and consists of over 1.4 million instances. It can serve as a basis for further research. The obtained dataset showed to be a valuable source for the creation of an error model, with which we could improve the ranking of candidate replacement words. This model takes the character context in which erroneous edit operations occur into account, and therefore reflects what kind of edit operations are more likely to occur. The spellchecking results using our dataset show that the context-based approach used, works for both the detection of errors and the ranking of candidate replacements. A comparison with literature was made to assess if the technique used performs as good for Dutch as for English and we conclude that the performance is comparable. The error model trained on our dataset was shown to work better than the context-based approach for the task of candidate ranking.Information ArchitectureWeb Information SystemsElectrical Engineering, Mathematics and Computer Scienc
A 0.12mm<sup>2</sup> Wien-Bridge Temperature Sensor with 0.1°C (3σ) Inaccuracy from -40°C to 180°C
No full text Resistor-based temperature sensors can achieve much higher resolution and energy efficiency than conventional BJT-based sensors [1], but they typically occupy more area (> 0.25 mm 2 ) and have lower operating temperatures (le 125 {circ} {C}) [2]-[4]. This work describes a 0.12mm 2 resistor-based sensor that uses a Wien-bridge (WB) filter to achieve 0.1 {circ} {C} (3 sigma) inaccuracy from - 40 {circ} {C} to 180 {circ} {C}. Compared to a state-of-the-art WB sensor [4], it occupies 6 × less area and achieves comparable relative accuracy over a 76% wider operating range. Session 10.3 Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Electronic InstrumentationMicroelectronic
Feather-pecking and injurious pecking in organic laying hens in 107 flocks from eight european countries
No full textFeather-pecking and cannibalism may reduce the potential of organic husbandry to enhance the welfare of laying hens. We report risk factors for these issues based on a large survey of 107 commercial flocks in eight European countries. Information was collected regarding housing, management and flock characteristics (age, genotype). Near the end of lay, 50 hens per flock were assessed for plumage condition and wounds. Potential influencing factors were screened and submitted to a multivariate model. The majority of the flocks (81%) consisted of brown genotypes and were found in six countries. Since white genotypes (19%) were found only in the two Scandinavian countries, a country effect could not be excluded. Therefore, separate models were made for brown and white genotypes. Feather damage in brown hens could be explained by a model containing a lower dietary protein content and no daily access to the free range (30% of the variation explained). For feather damage in white hens, no model could be made. Wounds in brown hens were associated with not having daily access to free range (14% of the variation explained). Wounds in white hens were explained by a model containing not topping-up litter during the laying period (26% of the variation explained). These results suggest that better feeding management, daily access to the free-range area and improved litter management may reduce incidence of plumage damage and associated injurious pecking, hence enhancing the welfare of organic laying hens. Since this was an epidemiological study, further experimental studies are needed to investigate the causal relationships
A ±25A Versatile Shunt-Based Current Sensor with 10kHz Bandwidth and ±0.25% Gain Error from -40°C to 85°C Using 2-Current Calibration
No full textAccurate current sensing is critical in many industrial applications, such as battery management and motor control. Precise shunt-based current sensors have been reported with gain errors of less than 1% over the industrial temperature range (-40°C to 85°C) [1]–[4]. However, since they are intended for coulomb counting, their bandwidth is limited to a few tens of Hz, making them unsuitable for battery impedance or motor-current sensing. This paper presents a current sensor with a wide (10kHz) bandwidth and a tunable temperature compensation scheme (TCS), which allows it to be flexibly used with different types of shunts while maintaining high accuracy. A low-cost room-temperature calibration scheme is proposed to optimize gain flatness over temperature by exploiting the shunt's self-heating at large currents. Over the industrial temperature range and a ±25A current range, it achieves state-of-the-art gain error (±0.25%) with both low-cost PCB and stable metal-alloy shunts.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Electronic InstrumentationMicroelectronic
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