Borko Stosic

Multifractal analysis of human retinal vessels

In this paper, it is shown that vascular structures of the human retina represent geometrical multifractals, characterized by a hierarchy of exponents rather then a single fractal dimension. A number of retinal images from the STARE database are analyzed, corresponding to both normal and pathological states of the retina. In all studied cases, a clearly multifractal behavior is observed, where capacity dimension is always found to be larger then the information dimension, which is in turn always larger then the correlation dimension, all the three being significantly lower then the diffusion limited aggregation (DLA) fractal dimension. We also observe a tendency of images corresponding to the pathological states of the retina to have lower generalized dimensions and a shifted spectrum range, in comparison with the normal cases

EPOTRAN: a full-differential Monte Carlo code for electron and positron transport in liquid and gaseous water.

Abstract Purpose: We describe here a novel full-differential Monte Carlo (MC) event-by-event simulation, for modelling electron and positron histories in liquid and gaseous water, with impact energies ranging from the water excitation threshold (7.4 eV) to 10 keV. This new track-structure code is named EPOTRAN, an acronym for Electron and POsitron TRANsport in water. Material and methods: All the processes induced by both electrons and positrons are studied in detail via theoretical differential and total cross sections, calculated within the quantum mechanical framework by using the partial-wave method. Elastic and inelastic interactions are then successively reviewed, including in particular an original description of the positron-induced capture process leading to Positronium formation. Results: Total and differential cross sections are reported and compared with a large set of existing measurements. Rather good agreement is generally observed over the considered energy range. Conclusions: This work reports the theoretical cross sections used in a special purpose Monte Carlo simulation suitable for electron and positron transport in gaseous and liquid water. This MC code should represent an accurate tool for dose calculation at the nanometric scale, by providing a detailed spatial distribution of energy deposits. Furthermore, positron trajectory studies made possible by this approach should prove useful for evaluating the real contribution of the positron range on the overall spatial resolution of PET (Positron Emission Tomography) imaging.

Closed form expressions for moments of the beta Weibull distribution

The beta Weibull distribution was first introduced by Famoye et al. (2005) and studied by these authors and Lee et al. (2007). However, they do not give explicit expressions for the moments. In this article, we derive explicit closed form expressions for the moments of this distribution, which generalize results available in the literature for some sub-models. We also obtain expansions for the cumulative distribution function and Renyi entropy. Further, we discuss maximum likelihood estimation and provide formulae for the elements of the expected information matrix. We also demonstrate the usefulness of this distribution on a real data set.

Relationship between bedload and suspended sediment in the sand-bed Exu River, in the semi-arid region of Brazil

Abstract Suspended sediment and bedload discharges in sand-bed rivers shape semi-arid landscapes and impact sediment delivery from these landscapes, but are still incompletely understood. Suspended sediment and bedload fluxes of the intermittent Exu River, Brazil, were sampled by direct measurements. The highest suspended sediment concentration observed was 4847.4 mg L-1 and this value was possibly associated with the entrainment of sediment that was deposited in the preceding year. The bedload flux was well related to the stream power and the river efficiently transported all available bedload with a mean rate of 0.0047 kg m-1 s-1, and the percentage of bedload to suspended sediment varied between 4 and 12.72. The bed sediment of Exu River was prone to entrainment and showed a proclivity for transport. Thus, sand-bed and gravel-bed rivers of arid environments seem to exhibit the same mobility in the absence of armour layer. Editor D. Koutsoyiannis; Associate editor B. Touaibia Citation Cantalice, J.R.B., Cunha Filho, M., Stosic, B.D., Piscoya, V.C., Guerra, S.M.S., and Singh, V.P., 2013. Relationship between bedload and suspended sediment in the sand-bed Exu River, in the semi-arid region of Brazil. Hydrological Sciences Journal, 58 (8), 1789–1802.

Anomalous behavior of the zero field susceptibility of the Ising model on the Cayley tree

It is found that the zero field susceptibility χ of the Ising model on the Cayley tree exhibits unusually weak divergence at the critical point TC. The susceptibility amplitude is found to diverge at TC proportionally to the tree generation level n, while the behavior of χ is otherwise analytic in the vicinity of TC,with the critical exponent γ=0.

Estabilidade em análise de agrupamento: estudo de caso em ciência florestal

The main objective of this research was to propose a system to the study and interpretation of stability in cluster analysis through several cluster algorithms in vegetation data. The data set used derived from a survey in the Silviculture Forest at Federal University of Vicosa - MG. To perform the cluster analysis, the Mahalanobis distance matrices were estimated on basis of original data and bootstrap resampling. Also, the single linkage, complete linkage, average distances, centroid, median and Ward methods were used. Chi-square test was applied to detect the association among the methods. A co-phenetic correlation was obtained for the cluster methods. The results for the method associations were very similar, indicating that any algorithm of the studied clusters is stabilized, and in fact, that groups exist among the analyzed individuals. However, it was verified that the methods are coincident, except for the centroid and Ward, and also the centroid and median methods, when compared to Ward, respectively, based on the Mahalanobis matrices derived from the original data set and bootstrap. The methodology proposed is promising to the study and interpretation of the stability of cluster analysis methods in vegetation data.

k-Means Algorithm in Statistical Shape Analysis

In this work it is shown how the k-means method for clustering objects can be applied in the context of statistical shape analysis. Because the choice of the suitable distance measure is a key issue for shape analysis, the Hartigan and Wong k-means algorithm is adapted for this situation. Simulations on controlled artificial data sets demonstrate that distances on the pre-shape spaces are more appropriate than the Euclidean distance on the tangent space. Finally, results are presented of an application to a real problem of oceanography, which in fact motivated the current work.

Morphological discrimination in juveniles of two Selene species (Teleostei: Carangidae) using truss net distances

External morphological distinction between young Selene vomer and S. brownii (Carangidae) was studied using multivariate analysis with the purpose to evaluate if other characters than the second dorsal and pelvic fin spines could be used as distinguishing attributes for juveniles fishes. These spines are important distinctive morphological characters, but they can be absent in juvenile fishes or be broken during manipulation. A sample of 389 specimens was collected from beach netting at Caicara do Norte (RN), northeastern Brazil, in 2001. Each specimen was digitalized and their images provided numerical matrices using Scion Image and FANTOMAS software. A C-language program was developed and used to calculate 16 Euclidean distances and nine homologous landmarks plotted on images. Discriminant analysis results revealed that the two Selene species was clearly distinguished employing morphometric procedures of diagonally oriented measurements of the antero-posterior body axis.

Power law correlations in time series of wild-land and forest fires in Brazil

We study temporal correlations in daily hot pixel time series detected in Brazil in the period 1998–2007, using detrended fluctuation analysis (DFA). We find that power law correlations decay following two distinct scaling regimes with exponents a 1 ≅ 0.8 and a 2 ≅ 1.3, with a crossover point at approximately 45 days. These results agree with previous studies of burned area time series of forest fires in other regions of the world and should be taken into account when modelling wild-land and forest fires in Brazil.

Computational Approach to Improving the Efficiency of River Discharge Measurement

AbstractIn this work, a simple computational scheme with the pragmatic purpose of augmenting the efficiency of river discharge measurements is proposed. Observing the functional form of the velocity profile versus depth, applying polynomial regression for each vertical, and finally interpolating between the verticals, the authors develop a continuous approximation for the velocity across the entire vertical section, which turns out to be rather robust to the removal of individual measurement points. In particular, numerical data analysis shows that the number of measurements can be significantly reduced without a significant loss of accuracy of the discharge estimate. This procedure has been tested on the data obtained experimentally on the Exu, Capibaribe, and Ipojuca rivers, in the state of Pernambuco, northeast of Brazil, with different streamflow patterns exhibiting multiple local velocity maxima on and below the surface.