16,685 research outputs found
Discrete Dynamical Systems: A Brief Survey
Dynamical system is a mathematical formalization for any fixed rule that is described in time dependent fashion. The time can be measured by either of the number systems - integers, real numbers, complex numbers. A discrete dynamical system is a dynamical system whose state evolves over a state space in discrete time steps according to a fixed rule. This brief survey paper is concerned with the part of the work done by José Sousa Ramos [2] and some of his research students. We present the general theory of discrete dynamical systems and present results from applications to geometry, graph theory and synchronization
Superlens made of a metamaterial with extreme effective parameters
We propose a superlens formed by an ultra-dense array of crossed metallic
wires. It is demonstrated that due to the anomalous interaction between crossed
wires, the structured substrate is characterized by an anomalously high index
of refraction and supports strongly confined guided modes with very short
propagation wavelengths. It is theoretically proven that a planar slab of such
structured material makes a superlens that may compensate for the attenuation
introduced by free-space propagation and restore the subwavelength details of
the source. The bandwidth of the proposed device can be quite significant since
the response of the structured substrate is non-resonant. The theoretical
results are fully supported by numerical simulations.Comment: Accepted for publication in Phys. Rev. B (in press
Modeling the input history of programs for improved instruction-memory performance
When a program is loaded into memory for execution, the relative position of
its basic blocks is crucial, since loading basic blocks that are unlikely to be
executed first places them high in the instruction-memory hierarchy only to be
dislodged as the execution goes on. In this paper we study the use of Bayesian
networks as models of the input history of a program. The main point is the
creation of a probabilistic model that persists as the program is run on
different inputs and at each new input refines its own parameters in order to
reflect the program's input history more accurately. As the model is thus
tuned, it causes basic blocks to be reordered so that, upon arrival of the next
input for execution, loading the basic blocks into memory automatically takes
into account the input history of the program. We report on extensive
experiments, whose results demonstrate the efficacy of the overall approach in
progressively lowering the execution times of a program on identical inputs
placed randomly in a sequence of varied inputs. We provide results on selected
SPEC CINT2000 programs and also evaluate our approach as compared to the gcc
level-3 optimization and to Pettis-Hansen reordering
Assessing consistency of fish survey data : uncertainties in the estimation of mackerel icefish (Champsocephalus gunnari) abundance at South Georgia
Acknowledgments The authors wish to thank the crews, fishermen and scientists who conducted the various surveys from which data were obtained, and Mark Belchier and Simeon Hill for their contributions. This work was supported by the Government of South Georgia and South Sandwich Islands. Additional logistical support provided by The South Atlantic Environmental Research Institute with thanks to Paul Brickle. Thanks to Stephen Smith of Fisheries and Oceans Canada (DFO) for help in constructing bootstrap confidence limits. Paul Fernandes receives funding from the MASTS pooling initiative (The Marine Alliance for Science and Technology for Scotland), and their support is gratefully acknowledged. MASTS is funded by the Scottish Funding Council (grant reference HR09011) and contributing institutions. We also wish to thank two anonymous referees for their helpful suggestions on earlier versions of this manuscript.Peer reviewedPostprin
BOND: Bayesian Oxygen and Nitrogen abundance Determinations in giant H II regions using strong and semi-strong lines
We present BOND, a Bayesian code to simultaneously derive oxygen and nitrogen
abundances in giant H II regions. It compares observed emission lines to a grid
of photoionization models without assuming any relation between O/H and N/O.
Our grid spans a wide range in O/H, N/O and ionization parameter U, and covers
different starburst ages and nebular geometries. Varying starburst ages
accounts for variations in the ionizing radiation field hardness, which arise
due to the ageing of H II regions or the stochastic sampling of the initial
mass function. All previous approaches assume a strict relation between the
ionizing field and metallicity. The other novelty is extracting information on
the nebular physics from semi-strong emission lines. While strong lines ratios
alone ([O III]/Hbeta, [O II]/Hbeta and [N II]/Hbeta) lead to multiple O/H
solutions, the simultaneous use of [Ar III]/[Ne III] allows one to decide
whether an H II region is of high or low metallicity. Adding He I/Hbeta pins
down the hardness of the radiation field. We apply our method to H II regions
and blue compact dwarf galaxies, and find that the resulting N/O vs O/H
relation is as scattered as the one obtained from the temperature-based method.
As in previous strong-line methods calibrated on photoionization models, the
BOND O/H values are generally higher than temperature-based ones, which might
indicate the presence of temperature fluctuations or kappa distributions in
real nebulae, or a too soft ionizing radiation field in the models.Comment: MNRAS in press; 21 pages, 22 figures, 2 tables; code, data and
results available at http://bond.ufsc.b
A Caldeira do Fogo : génese e evolução das formas do relevo
Projecto de investigação apresentado à Universidade dos Açores para obtenção do grau de doutor em Geografia.Prova complementar de doutoramento.Olhado no passado como peça fundamental na contextura da expansão europeia e, no presente, como fonte inesgotável de recursos, o mar foi, e ainda é, um obstáculo de declarada importância no desenvolvimento económico dos Açores, porque quebra a continuidade do espaço territorial do arquipélago, porque o torna longe dos dois continentes mais próximos e porque condiciona a ocupação humana à escassez do chão insular. Para uma sociedade de tradições agro-pastoris como a açoriana que do amanho da terra retira a sua maior riqueza, o último aspecto referido reveste-se de particular acuidade. Não só as águas confinam as suas terras como, do eterno antagonismo entre o oceano e a terra firme, aquele sai sempre vitorioso. Outrossim, o mar é, por excelência, a bacia receptora dos solos que, no interior das ilhas, a erosão pouco a pouco vai roubando. O lento, mas persistente recuo da costa, é um processo natural que resulta da continuada agressividade das ondas: contra a poderosa força destas, pouca ou nenhuma oposição pode o habitante ilhéu oferecer. A erosão, tomada aqui como desgaste, é um processo de denudaçao dos continentes, natural também e tão antigo como estes o são. Neste campo porém, a interferência do homem pode revestir-se de aspectos significativos que visem retardar ou minimizar o mecanismo erosivo ou de outros que, pelo contrário criem condições ao disparar do processo ou a sua aceleração. No arquipélago dos Açores, e apesar do prejuízo irreparável que actualmente representa o transporte de camadas de solo pelas escorrência superficial, os efeitos da erosão têm sido descorados pelo homem ou olhados com alguma negligência. [...
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