893 resultados para finite-time blow-up


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This paper presents a finite-difference time-domain (FDTD) simulator for electromagnetic analysis and design applications in MRI. It is intended to be a complete FDTD model of an MRI system including all RF and low-frequency field generating units and electrical models of the patient. The pro-ram has been constructed in an object-oriented framework. The design procedure is detailed and the numerical solver has been verified against analytical solutions for simple cases and also applied to various field calculation problems. In particular, the simulator is demonstrated for inverse RF coil design, optimized source profile generation, and parallel imaging in high-frequency situations. The examples show new developments enabled by the simulator and demonstrate that the proposed FDTD framework can be used to analyze large-scale computational electromagnetic problems in modern MRI engineering. (C) 2004 Elsevier Inc. All rights reserved.

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We consider a problem of robust performance analysis of linear discrete time varying systems on a bounded time interval. The system is represented in the state-space form. It is driven by a random input disturbance with imprecisely known probability distribution; this distributional uncertainty is described in terms of entropy. The worst-case performance of the system is quantified by its a-anisotropic norm. Computing the anisotropic norm is reduced to solving a set of difference Riccati and Lyapunov equations and a special form equation.

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Mathematics Subject Classification 2010: 26A33, 33E12, 35S10, 45K05.

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A válság okozta megszorítások a projektek költségvetését sem hagyták változatlanul. Nagyon sokszor nemcsak a jövőbeni projekttervek költségvetését kell átgondolni, hanem a már futó projektek költségvetését is újra kell szabni. E tanulmány ilyen esetekben nyújthat módszertani támogatást. A szerző ebben a kutatásban négy költség- és időcsökkentő módszert hasonlít össze. Ismerteti, hogy ezeket az eljárásokat milyen módon lehet ötvözni, illetve mikor, melyiket célszerű alkalmazni. Az eljárások között van olyan módszer, amely a hagyományos projektmenedzsment (pl. építési, beruházási projektek menedzselésének) eszköztárát gazdagítja, de találkozhatunk olyan eljárásokkal is, amelyek az agilis projektszemléleten alapuló módszerek körét szélesítik. A bemutatott módszerek nemcsak a hálótervezési, hanem a mátrixos projekttervezési eljárások esetén is alkalmazhatók. ____ Due to the effects of the crisis, budgets of present as well as future projects are decreasing steadily. In this study four different methods are introduced for minimising budget and time demands. These introduced methods support not only the traditional but also the agile project management. Furthermore these methods can be used not only in case of network planning, but also for matrix-based project planning.

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The current mobile networks don't offer sufficient data rates to support multimedia intensive applications in development for multifunctional mobile devices. Ultra wideband (UWB) wireless technology is being considered as the solution to overcome data rate bottlenecks in the current mobile networks. UWB is able to achieve such high data transmission rates because it transmits data over a very large chunk of the frequency spectrum. As currently approved by the U.S. Federal Communication Commission it utilizes 7.5 GHz of spectrum between 3.1 GHz and 10.6 GHz. ^ Successful transmission and reception of information data using UWB wireless technology in mobile devices, requires an antenna that has linear phase, low dispersion and a voltage standing wave ratio (VSWR) ≤ 2 throughout the entire frequency band. Compatibility with an integrated circuit requires an unobtrusive and electrically small design. The previous techniques that have been used to optimize the performance of UWB wireless systems, involve proper design of source pulses for optimal UWB performance. The goal of this work is directed towards the designing of antennas for personal communication devices, with optimal UWB bandwidth performance. Several techniques are proposed for optimal UWB bandwidth performance of the UWB antenna designs in this Ph.D. dissertation. ^ This Ph.D. dissertation presents novel UWB antenna designs for personal communication devices that have been characterized and optimized using the finite difference time domain (FDTD) technique. The antenna designs reported in this research are physically compact, planar for low profile use, with sufficient impedance bandwidth (>20%), antenna input impedance of 50-Ω, and an omni-directional (±1.5 dB) radiation pattern in the operating bandwidth. ^

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Since the 1950s, the theory of deterministic and nondeterministic finite automata (DFAs and NFAs, respectively) has been a cornerstone of theoretical computer science. In this dissertation, our main object of study is minimal NFAs. In contrast with minimal DFAs, minimal NFAs are computationally challenging: first, there can be more than one minimal NFA recognizing a given language; second, the problem of converting an NFA to a minimal equivalent NFA is NP-hard, even for NFAs over a unary alphabet. Our study is based on the development of two main theories, inductive bases and partials, which in combination form the foundation for an incremental algorithm, ibas, to find minimal NFAs. An inductive basis is a collection of languages with the property that it can generate (through union) each of the left quotients of its elements. We prove a fundamental characterization theorem which says that a language can be recognized by an n-state NFA if and only if it can be generated by an n-element inductive basis. A partial is an incompletely-specified language. We say that an NFA recognizes a partial if its language extends the partial, meaning that the NFA’s behavior is unconstrained on unspecified strings; it follows that a minimal NFA for a partial is also minimal for its language. We therefore direct our attention to minimal NFAs recognizing a given partial. Combining inductive bases and partials, we generalize our characterization theorem, showing that a partial can be recognized by an n-state NFA if and only if it can be generated by an n-element partial inductive basis. We apply our theory to develop and implement ibas, an incremental algorithm that finds minimal partial inductive bases generating a given partial. In the case of unary languages, ibas can often find minimal NFAs of up to 10 states in about an hour of computing time; with brute-force search this would require many trillions of years.

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Antenna design is an iterative process in which structures are analyzed and changed to comply with certain performance parameters required. The classic approach starts with analyzing a "known" structure, obtaining the value of its performance parameter and changing this structure until the "target" value is achieved. This process relies on having an initial structure, which follows some known or "intuitive" patterns already familiar to the designer. The purpose of this research was to develop a method of designing UWB antennas. What is new in this proposal is that the design process is reversed: the designer will start with the target performance parameter and obtain a structure as the result of the design process. This method provided a new way to replicate and optimize existing performance parameters. The base of the method was the use of a Genetic Algorithm (GA) adapted to the format of the chromosome that will be evaluated by the Electromagnetic (EM) solver. For the electromagnetic study we used XFDTD™ program, based in the Finite-Difference Time-Domain technique. The programming portion of the method was created under the MatLab environment, which serves as the interface for converting chromosomes, file formats and transferring of data between the XFDTD™ and GA. A high level of customization had to be written into the code to work with the specific files generated by the XFDTD™ program. Two types of cost functions were evaluated; the first one seeking broadband performance within the UWB band, and the second one searching for curve replication of a reference geometry. The performance of the method was evaluated considering the speed provided by the computer resources used. Balance between accuracy, data file size and speed of execution was achieved by defining parameters in the GA code as well as changing the internal parameters of the XFDTD™ projects. The results showed that the GA produced geometries that were analyzed by the XFDTD™ program and changed following the search criteria until reaching the target value of the cost function. Results also showed how the parameters can change the search criteria and influence the running of the code to provide a variety of geometries.

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The United States has over 4 million births annually. Currently healthy women with non-complicated deliveries receive little to no routine postpartum support when discharged from the hospital. This is especially problematic if mothers are first time mothers, poor, have language barriers and little to no social support after giving birth. The purpose of this randomized clinical trial was to compare maternal and infant health outcomes, and health care charges between 2 groups of mothers and newborns. A control ( n = 69) group received routine posthospital discharge care. An intervention group (n = 70) received routine posthospital discharge care plus follow up telephone calls by advanced practice nurses (APNs) on days 3,7,14,21,28 and week 8. Both groups were followed for the first 8 weeks posthospital discharge following delivery to examine maternal health outcomes (perceived maternal stress, social support and perceived maternal physical health), infant health outcomes (routine medical follow up visits immunizations, weight gain), morbidity (urgent care visits, emergency room visits, rehospitalizations), health care charges (urgent care visits, emergency room visits, rehospitalizations) in both groups and charges for APN follow up in the intervention group only. Data were analyzed using descriptive statistics and two-sample t-tests. Study findings indicated that intervention group had significantly lower perceived maternal stress, significantly higher rating of perceived maternal health and higher levels of social support and by the end of the 2nd month posthospital discharge compared to control group mothers. Infants in the intervention group had: increased number of immunizations; fewer emergency room visits; and 1 infant rehospitalization compared to 3 infant rehospitalizations in the control group. The intervention groups' health care charges were significantly lower compared to the control group $14,333/$497 vs. $70,834/$1,068. These study results indicate that an intervention of APN follow up telephone calls in this sample of first time low-income culturally diverse mothers was an effective, safe, low cost, easy to apply intervention which improved mothers' and infants' health outcomes and reduced healthcare charges.