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Continuous-time systems are usually modelled by differential equations arising from physical laws. However, the use of these models in practice requires discretisation. In this thesis we consider sampled-data models for linear and nonlinear systems. We study some of the issues involved in the sampling process, such as the accuracy of the sampled-data models, the artifacts produced by the particular sampling scheme, and the relations to the underlying continuous-time system. We review, extend and present new results, making extensive use of the delta operator which allows a clearer connection between a sampled-data model and the underlying continuous-time system. In the first part of the thesis we consider sampled-data models for linear systems. In this case exact discrete-time representations can be obtained. These models depend, not only on the continuous-time system, but also on the artifacts involved in the sampling process, namely, the sample and hold devices. In particular, these devices play a key role in determining the sampling zeros of the discrete-time model. We consider robustness issues associated with the use of discrete-time models for continuous-time system identification from sampled data. We show that, by using restricted bandwidth frequency domain maximum likelihood estimation, the identification results are robust to (possible) under-modelling due to the sampling process. Sampled-data models provide a powerful tool also for continuous-time optimal control problems, where the presence of constraints can make the explicit solution impossible to find. We show how this solution can be arbitrarily approximated by an associated sampled-data problem using fast sampling rates. We also show that there is a natural convergence of the singular structure of the optimal control problem from discrete- to continuous-time, as the sampling period goes to zero. In Part II we consider sampled-data models for nonlinear systems. In this case we can only obtain approximate sampled-data models. These discrete-time models are simple and accurate in a well defined sense. For deterministic systems, an insightful observation is that the proposed model contains sampling zero dynamics. Moreover, these correspond to the same dynamics associated with the asymptotic sampling zeros in the linear case. The topics and results presented in the thesis are believed to give important insights into the use of sampled-data models to represent linear and nonlinear continuous-time systems. / PhD Doctorate

The emergence of new data models requires further research into federated database systems. A federated database system (FDBS) provides uniform access to multiple heterogeneous databases. Most FDBS's provide access to only the older data models such as relational, hierarchical, and network models. A federated system requires a meta data model. The meta model is a uniform data model through which users access data regardless of the data model of the data's native database. This dissertation examines the question of meta models for use in an FDBS that provides access to relational, object oriented, and rule based databases. This dissertation proposes Cyrano, a hybrid of object oriented and rule based data models. The dissertation demonstrates that Cyrano is suitable as a meta model by showing that Cyrano satisfies the following three criteria: 1) Cyrano fully supports relational, object oriented, and rule based member data models. 2) Cyrano provides sufficient capabilities to support integration of heterogeneous databases. 3) Cyrano can be implemented as the meta model of an operational FDBS. This dissertation describes four primary products of this research: 1) The dissertation presents Cyrano, a meta model designed as part of this research that supports both the older and the newer data models. Cyrano is an example of analytic object orientation. Analytic object orientation is a conceptual approach that combines elements of object oriented and rule based data models. 2) The dissertation describes Roxanne, a proof-of-concept FDBS that uses Cyrano as its meta model. 3) The dissertation proposes a set of criteria for the evaluation of meta models. The dissertation uses these criteria to demonstrate Cyrano's Suitability as a meta model. 4) The dissertation presents an object oriented FDBS reference architecture suitable for use in describing and designing an FDBS. / Ph. D.

The problems of design, operation, and maintenance of databases using the three most popular database management systems (Hierarchical, CQDASYL/DBTG, and Relational) are well known. Users wishing to use these systems have to make conscious and often complex mappings between the real-world structures and the data structuring options (data models) provided by these systems. In addition, much of the semantics associated with the data either does not get expressed at all or gets embedded procedurally in application programs in an ad-hoc way. In recent years, a large number of data models (called semantic data models) have been proposed with the aim of simplifying database design and use. However, the lack of usable implementations of these proposals has so far inhibited the widespread use of these concepts. The present work reports on an effort to evaluate and extend one such semantic model by means of an implementation. It is based on the functional data model proposed earlier by Shipman (SHIP81). We call this 'Extended Functional Data Model' (EFDM). EFDM, like Shipman's proposals, is a marriage of three of the advanced modelling concepts found in both database and artificial intelligence research: the concept of entity to represent an object in the real world, the concept of type hierarchy among entity types, and the concept of derived data for modelling procedural knowledge. The functional notation of the model lends itself to high level data manipulation languages. The data selection in these languages is expressed simply as function application. Further, the functional approach makes it possible to incorporate general purpose computation facilities in the data languages without having to embed them in procedural languages. In addition to providing the usual database facilities, the implementation also provides a mechanism to specify multiple user views of the database.

During the development of federated IS that make use of non-homogenous databases and data sources, XML documents are often used for data exchange among the local subsystems, while their corresponding XML Schemas are generated using the standard CASE tools for local systems. External data schemes of those systems must be specified in a unified common model. An assumption is given that OBJECT PROPERTY (OP) model is being used for the semantic integration of the local non-homogenous subsystems. A graphical editor was developed that can be used to specify relation objects, their identifiers, complex and multi-valued object attributes. As OP model’s semantic expression capabilities can map those available in XML, additionally rules have been defined and implemented that can transform specific OP model structures into XML Schemas. Also algorithm is specified that can be used to extract tree-like structures from the model.Example transformations are performed that illustrate the process of generation of XML Schemas documents from sample OP models.

This research is an empirical investigation of human factors on the use of database systems. The problem motivating the study is the difficulty encountered by end-users in retrieving data from a database.

This thesis aims to demonstate Andoid game development principles using an sample ap-plicatiion. The practical part of this work is a standalone implementation of a Java-based text adventure game wirh SQLite for game-data storage. A game framework has been de-veloped which enables you to enter various game-related data and thus play different ga-mes based on the specifics of the input.

Após três décadas de pesquisa em hipermídia, muitos problemas identificados ainda não foram totalmente solucionados. Problemas relacionados com desorientação, sobrecarga cognitiva, qualidade de interface, interatividade e estruturação dos componentes dos sistemas hipermídia, são alguns que merecem ser citados. Particularmente, o problema clássico de desorientação do usuário recebeu maior atenção. Várias propostas de solução já foram sugeridas: use de roteiros guiados em substituição a navegação, retorno ao no anterior (`backtrack'), histórico de nos visitados, marcas de livro (`bookmarks'), mapas globais e locais, visões olho-de-peixe, metáforas, folheadores (`browsers'), entre outras. Se observa também que o avanço tecnológico permite que cada vez mais as aplicações incluam dados multimídia. Esta tendência mostra a necessidade urgente do surgimento de novas técnicas de modelagem de aplicações hipermídia que diminuam os problemas citados anteriormente. Desta forma, se pretende com a presente dissertação, propor uma técnica de modelagem de aplicações hipermídia, capaz de diminuir os problemas de desorientação do usuário e também de facilitar a identificação das estruturas compreensíveis que interligarão os componentes da aplicação. A técnica de modelagem aqui proposta - HMT (`Hypermedia Modeling Technique') - utiliza quatro modelos para descrever uma aplicação: o modelo de objetos descreve os objetos do domínio da aplicação e seus relacionamentos; o modelo de hiperobjetos refina o modelo de objetos, adicionando maior semântica aos relacionamentos; o modelo de navegação descreve os elos e estruturas de acesso e o modelo de interface contem as descrições sobre como o usuário ira perceber os objetos hipermídia. A técnica HMT se baseou no levantamento dos problemas relevantes as aplicações hipermídia e principalmente na analise das propostas e dos principais trabalhos encontrados na literatura. Finalmente, reforçando a viabilidade das idéias, foi feita a modelagem, projeto e implementação da aplicação hipermídia sobre literatura no Rio Grande do Sul, sob a forma de CD-ROM : Enciclopédia da Literatura Rio-Grandense. / After three decades of hypermedia research a great number of identified problems still remain without a total solution. Problems related to disorientation, cognitive overhead, interface, interaction and structure of hypermedia applications components are some of the main problems. The classical user disorientation problem has been the main focus of the attention. Many solution proposals have been suggested: the use of guided tours replacing navigation, backtrack, history mechanisms, bookmarks, global and local maps, fisheye views, metaphors, browsers, among others. We note that the technological advance allows the construction of applications that include multimedia data. This shows us that new modeling techniques for hypermedia applications are required to reduce the problems cited above. In this dissertation, we intend to propose a modeling technique for hypermedia applications, capable of reducing both the user disorientation problem and the identification of comprehensible structures that will link the components of the application. This modeling technique - the HMT (Hypermedia Modeling Technique) - uses four models to describe an application: the object model describes the objects of the application domain and their relationships; the hyperobject model enhances the object model, adding more semantics to the relationships; the navigation model describes the access structures and the interface model contains the descriptions of how the user will perceive the hypermedia objects. The HMT technique was based on the relevant problems related to hypermedia applications and mainly on the analysis of the proposals and related research found in the literature. Finally, reinforcing the viability of the proposed ideas, an application was modeled, designed and implemented: the CD-ROM "Enciclopedia da Literatura Rio- Grandense" which deals with literary information.

The thesis contains the four chapters: Testing parameter constancy in unit root autoregressive models against continuous change; Dickey-Fuller type of tests against nonlinear dynamic models; Inference for unit roots in a panel smooth transition autoregressive model where the time dimension is fixed; Testing unit roots in nonlinear dynamic heterogeneous panels. In Chapter  1 we derive tests for parameter constancy when the data generating process is non-stationary against the hypothesis that the parameters of the model change smoothly over time. To obtain the asymptotic distributions of the tests we generalize many theoretical results, as well as new are introduced, in the area of unit roots . The results are derived under the assumption that the error term is a strong mixing. Small sample properties of the tests are investigated, and in particular, the power performances are satisfactory. In Chapter 2 we introduce several test statistics of testing the null hypotheses of a random walk (with or without drift) against models that accommodate a smooth nonlinear shift in the level, the dynamic structure, and the trend. We derive analytical limiting distributions for all tests. Finite sample properties are examined. The performance of the tests is compared to that of the classical unit root tests by Dickey-Fuller and Phillips and Perron, and is found to be superior in terms of power. In Chapter 3 we derive a unit root test against a Panel Logistic Smooth Transition Autoregressive (PLSTAR). The analysis is concentrated on the case where the time dimension is fixed and the cross section dimension tends to infinity. Under the null hypothesis of a unit root, we show that the LSDV estimator of the autoregressive parameter in the linear component of the model is inconsistent due to the inclusion of fixed effects. The test statistic, adjusted for the inconsistency, has an asymptotic normal distribution whose first two moments are calculated analytically. To complete the analysis, finite sample properties of the test are examined. We highlight scenarios under which the traditional panel unit root tests by Harris and Tzavalis have inferior or reasonable power compared to our test. In Chapter 4 we present a unit root test against a non-linear dynamic heterogeneous panel with each country modelled as an LSTAR model. All parameters are viewed as country specific. We allow for serially correlated residuals over time and heterogeneous variance among countries. The test is derived under three special cases: (i) the number of countries and observations over time are fixed, (ii) observations over time are fixed and the number of countries tend to infinity, and (iii) first letting the number of observations over time tend to infinity and thereafter the number of countries. Small sample properties of the test  show modest size distortions and satisfactory power being superior to the Im, Pesaran and Shin t-type of test. We also show clear improvements in power compared to a univariate unit root test allowing for non-linearities under the alternative hypothesis. / Diss. Stockholm : Handelshögskolan, 2004

This dissertation applies economic theories and econometric methods to analyze the interactions between government policies and economic agents in two important and current topics: the protection of the environment and illegal migration.Following the introduction, the second chapter studies the empirical strength of bi-directional linkages between environmental standards and performance, on the one hand, and environmental innovation, on the other. Our empirical results reveal that environmental R&D both spurs the tightening of government environmental standards and is spurred by the anticipation of such tightening, suggesting that U.S. environmental policy (at least in the context of the manufacturing industries that we study) has been responsive to innovation and effective in inducing innovation.The third chapter studies whether a voluntary reduction pollution programs can prompt firms to develop new environmental technologies that yield future emission reduction benefits. Conversely, a VRP may induce a participating firm to divert resources from environmental research to environmental monitoring and compliance activities that yield short-term benefits in reduced emissions. We find evidence that higher rates of program participation are associated with significant reductions in the number of successful environmental patent applications four to six years after the program ended.The fourth chapter examines the migration duration of Mexican immigrants in the U.S. using data from the Mexican Migration Project (MMP). In the past, temporary migrations were frequent, and often the rule rather than the exception in the case of Mexican immigrants. This pattern may be changing due to the tightening of the border between Mexico and the Unites States. Moreover, this paper examines whether migration experience, demographic characteristics, economic conditions or social networks drive the time Mexican immigrants to reside illegally in the United States. The empirical analysis shows that the migration duration increases as the U.S. expected real wage increases. Tighter U.S. migration policies have an ambiguous effect on the migration duration while longer distances decrease the hazard of return to their state of origin.In the final chapter of this dissertation, the general findings are concluded and some future avenues of research are discussed.