Página 1 dos resultados de 287 itens digitais encontrados em 0.007 segundos

Percolation in a network with long-range connections: Implications for cytoskeletal structure and function

Silveira, Paulo S P; Alencar, Adriano Mesquita; MAJUMDAR, Arnab; Lemos, Miriam; FREDBERG, Jeffrey J.; SUKI, Bela
Fonte: ELSEVIER SCIENCE BV Publicador: ELSEVIER SCIENCE BV
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
25.99%
Cell shape, signaling, and integrity depend on cytoskeletal organization. In this study we describe the cytoskeleton as a simple network of filamentary proteins (links) anchored by complex protein structures (nodes). The structure of this network is regulated by a distance-dependent probability of link formation as P = p/d(s), where p regulates the network density and s controls how fast the probability for link formation decays with node distance (d). It was previously shown that the regulation of the link lengths is crucial for the mechanical behavior of the cells. Here we examined the ability of the two-dimensional network to percolate (i.e. to have end-to-end connectivity), and found that the percolation threshold depends strongly on s. The system undergoes a transition around s = 2. The percolation threshold of networks with s < 2 decreases with increasing system size L, while the percolation threshold for networks with s > 2 converges to a finite value. We speculate that s < 2 may represent a condition in which cells can accommodate deformation while still preserving their mechanical integrity. Additionally, we measured the length distribution of F-actin filaments from publicly available images of a variety of cell types. In agreement with model predictions...

Statistical analysis and modeling of optical transport networks; Análise estatística e modelação de redes óticas de transporte

Routray, Sudhir Kumar
Fonte: Universidade de Aveiro Publicador: Universidade de Aveiro
Tipo: Tese de Doutorado
ENG
Relevância na Pesquisa
46.51%
Statistical analysis and modeling of networks is now an integral part of network science and engineering. In case of optical transport networks (OTNs), it can be used for the planning and dimensioning when the complete information is not available or is difficult to process. The core networks around the world today are almost optical and they form the backbone of the Internet. Therefore, the statistical characteristics of these networks must be studied to understand their nature and to estimate their parameters. In science and technology, network analysis and modeling are used for several purposes such as the analysis of their stability, reliability and long term evolution. Knowledge of the statistical models helps in the estimation of several critical parameters of the networks. The work presented in this thesis is focused on the analysis and modeling of link lengths and shortest path lengths in OTNs. The parameters used in the models presented in this thesis can be estimated from the very basic information of the networks such as the coverage area and the number of nodes, both of which can be found from the node locations. These models can be applied to estimate key parameters of the networks. In this thesis, we have shown that the link lengths of the OTNs follow general extreme value distribution. The parameters of the proposed distribution can be estimated from the average link lengths of the networks. We develop expressions for the average link lengths of OTNs which can be estimated with an average error of just 11%. We apply the developed model to estimate link length dependent parameters in OTNs. We show that the shortest path lengths of the OTNs follow Johnson SB distribution. We estimate the parameters of the developed model from the convex area and the number of nodes of the network. We also apply this model to estimate several shortest path-dependent parameters in OTNs.; A análise estatística e modelação de redes é atualmente uma parte integrante da ciência e engenharia de redes. No caso das redes óticas de transporte (OTN)...

The measurement and dynamic implications of thin filament lengths in heart muscle.

Robinson, T F; Winegrad, S
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /01/1979 EN
Relevância na Pesquisa
26.13%
1. The lengths of the thin filaments in amphibian and mammalian cardiac muscle have been determined from electron micrographs of serial transverse sections. Thin filament lengths in frog atrial trabeculae range from 0.8 to greater than 1.3 micrometers, with a maximum possible error of 0.14--0.15 micrometer. In rat atrial tissue the span is from 0.6 to more than 1.1 micrometer, whereas in rat papillary muscle the breadth of the distribution is much narrower, from 0.9 to greater than 1.1 micrometer. Double overlap of thin filaments should, therefore, exist over a wide range of sarcomere lenghts. Thin filaments from opposite halves of a sarcomere accommodate each other by flexing up to an angle of about 2 degrees and moving from the trigonal position among the thick filaments to the centre of the region between two thick filaments. Such rearrangement probably contributes to the internal resistance to shortening in the muscle. 2. Except for the variation in thin filament lengths, the over-all morphology of the cardiac sarcomere is generally similar to that found in skeletal muscle. Thick filaments in heart muscle are uniform in length, and their profiles change along their lengths. They are generally round in the M band, triangular adjacent to the M band...

Efficient Sparse Signal Transmission over a Lossy Link Using Compressive Sensing

Wu, Liantao; Yu, Kai; Cao, Dongyu; Hu, Yuhen; Wang, Zhi
Fonte: MDPI Publicador: MDPI
Tipo: Artigo de Revista Científica
Publicado em 13/08/2015 EN
Relevância na Pesquisa
25.99%
Reliable data transmission over lossy communication link is expensive due to overheads for error protection. For signals that have inherent sparse structures, compressive sensing (CS) is applied to facilitate efficient sparse signal transmissions over lossy communication links without data compression or error protection. The natural packet loss in the lossy link is modeled as a random sampling process of the transmitted data, and the original signal will be reconstructed from the lossy transmission results using the CS-based reconstruction method at the receiving end. The impacts of packet lengths on transmission efficiency under different channel conditions have been discussed, and interleaving is incorporated to mitigate the impact of burst data loss. Extensive simulations and experiments have been conducted and compared to the traditional automatic repeat request (ARQ) interpolation technique, and very favorable results have been observed in terms of both accuracy of the reconstructed signals and the transmission energy consumption. Furthermore, the packet length effect provides useful insights for using compressed sensing for efficient sparse signal transmission via lossy links.

Multiple-antenna systems in ad-hoc wireless networks

Govindasamy, Siddhartan, 1975-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 229 leaves
ENG
Relevância na Pesquisa
25.99%
The increasing demand for wireless communication services has resulted in crowding of the electromagnetic spectrum. The "spectral-commons" model, where a portion of the electromagnetic spectrum is public and used on an ad-hoc basis, has been proposed to free up spectrum that has been allocated but underutilized. Ad-hoc wireless networks (networks with no central control) are also interesting in their own right as they do not require costly infrastructure, are robust to single-node failures, and can be deployed in environments where it is difficult to deploy infrastructure. The main contributions of this thesis are expressions for the mean and in some cases the variance of the spectral efficiency (bits/second/Hz) of single-hop links in random wireless networks as a function of the number of antennas per node, link-length, interferer density, and path-loss-exponent (an environmental parameter that determines signal decay with distance), under assumptions chosen for realistic implementability in the near future. These results improve our understanding of such systems as they indicate the data rates achievable as a function of tangible parameters like user density and environmental characteristics, and are useful for designers of wireless networks to trade-off hardware costs...

Distributed shortest path algorithms for computer networks

Yen, Jin Y.
Fonte: Monterey, California. Naval Postgraduate School Publicador: Monterey, California. Naval Postgraduate School
Tipo: Relatório Formato: NA
EN_US
Relevância na Pesquisa
25.99%
This paper presents two distributed algorithms for finding shortest paths from a source node to all other nodes in an N-node network. These algorithms are executed at individual nodes using only local information. Algorithm 1 works in networks where there are no topological changes such as link failures, link recoveries or changes of link lengths. Algorithm 2 is a mofification of Algorithm 1 for networks where there are topological changes. Algorithm 1 determines the optimal shortest paths in at most N3/4 steps, which is only one-half of the computational upper bounds of Abram and Rhodes' and Segall, Merlin and Gallager's algorithms. After the last topological change, Algorithm 2 determines the optimal shortest paths in the same number of steps as Algorithm 1. There are many situations where the present algorithms will work up to N/2 times faster than the algorithms proposed by these authors; Prepared for: Chief of Naval Research Arlington, VA; http://archive.org/details/distributedshort00yenj; N0001479WR90027; NA

Slope lengths and generalized augmented links

Purcell, Jessica S.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.09%
In this paper, we determine geometric information on slope lengths of a large class of knots in the 3-sphere, based only on diagrammatical properties of the knots. In particular, we show such knots have meridian length strictly less than 4, and we find infinitely many families with meridian length approaching 4 from below. Finally, we present an example to show that, in contrast to the case of the regular augmented link, longitude lengths of these knots cannot be determined by a function of the number of twist regions alone.; Comment: v2: 20 pages, 13 figures. Simplified proofs of main results and added two sections, one giving examples of knots with meridian lengths approaching the upper bound of 4, and one showing that there are no bounds on longitude length in terms of twist number. Updated the title to reflect these changes. To appear Comm. Anal. Geom

2d quantum gravity with discrete edge lengths

Bittner, E.; Markum, H.; Riedler, J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 17/09/1998
Relevância na Pesquisa
36.31%
An approximation of the Standard Regge Calculus (SRC) was proposed by the $Z_2$-Regge Model ($Z_2$RM). There the edge lengths of the simplicial complexes are restricted to only two possible values, both always compatible with the triangle inequalities. To examine the effect of discrete edge lengths, we define two models to describe the transition from the $Z_2$RM to the SRC. These models allow to choose the number of possible link lengths to be $n = {4,8,16,32,64,...}$ and differ mainly in the scaling of the quadratic link lengths. The first extension, the $X^1_n$-Model, keeps the edge lengths limited and still behaves rather similar to the "spin-like" $Z_2$RM. The vanishing critical cosmological constant is reproduced by the second extension, the $X^C_n$-Model, which allows for increasing edge lengths. In addition the area expectation values are consistent with the scaling relation of the SRC.; Comment: 3 pages, 4 figures, contribution to LATTICE'98, to be published in Nucl. Phys. B (Proc. Suppl.)

Ising-link Quantum Gravity

Fleming, Tom; Gross, Mark; Renken, Ray
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 04/01/1994
Relevância na Pesquisa
36.16%
We define a simplified version of Regge quantum gravity where the link lengths can take on only two possible values, both always compatible with the triangle inequalities. This is therefore equivalent to a model of Ising spins living on the links of a regular lattice with somewhat complicated, yet local interactions. The measure corresponds to the natural sum over all 2^links configurations, and numerical simulations can be efficiently implemented by means of look-up tables. In three dimensions we find a peak in the ``curvature susceptibility'' which grows with increasing system size. However, the value of the corresponding critical exponent as well as the behavior of the curvature at the transition differ from that found by Hamber and Williams for the Regge theory with continuously varying link lengths.; Comment: 11 pages

The (n)-solvable filtration of the link concordance group and Milnor's invariants

Otto, Carolyn
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/11/2012
Relevância na Pesquisa
26.28%
We establish several new results about both the (n)-solvable filtration, F_n^m, of the set of link concordance classes and the (n)-solvable filtration of the string link concordance group. We first establish a relationship between Milnor's invariants and links, L, with certain restrictions on the 4-manifold bounded by M_L. Using this relationship, we can relate (n)-solvability of a link (or string link) with its Milnor's invariants. Specifically, we show that if a link is (n)-solvable, then its Milnor's invariants vanish for lengths up to 2^{n+2}-1. Previously, there were no known results about the "other half" of the filtration, namely F_{n.5}^m/F_{n+1}^m. We establish the effect of the Bing doubling operator on (n)-solvability and using this, we show that F_{n.5}^m/F_{n+1}^m is nontrivial for links (and string links) with sufficiently many components. Moreover, we show that these quotients contain an infinite cyclic subgroup. We also show that links and string links modulo (1)-solvability is a nonabelian group. We show that we can relate other filtrations with Milnor's invariants. We show that if a link is n-positive, then its Milnor's invariants will also vanish for lengths up to 2^{n+2}-1. Lastly, we prove that the Grope filtration...

Time evolution of link length distribution in PRL collaboration network

Sen, Parongama; Chandra, Anjan Kumar; Hajra, Kamalika Basu; Das, Pratap Kumar
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
35.99%
An important aspect of a Euclidean network is its link length distribution, studied in a few real networks so far. We compute the distribution of the link lengths between collaborators whose papers appear in the PhysicalReview Letters (PRL) in several years within a range of four decades. The distribution is non-monotonic; there is a peak at nearest neighbour distances followed by a sharp fall and a subsequent rise at larger distances. The behaviour of the statistical properties of the distribution with time indicates that collaborations might become distance independent in about thirty to forty years.; Comment: References added, paper shortened and modified

On Wireless Link Scheduling and Flow Control

Gore, Ashutosh Deepak
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/12/2008
Relevância na Pesquisa
26.04%
This thesis focuses on link scheduling in wireless mesh networks by taking into account physical layer characteristics. The assumption made throughout is that a packet is received successfully only if the Signal to Interference and Noise Ratio (SINR) at the receiver exceeds the communication threshold. The thesis also discusses the complementary problem of flow control. (1) We consider various problems on centralized link scheduling in Spatial Time Division Multiple Access (STDMA) wireless mesh networks. We motivate the use of spatial reuse as performance metric and provide an explicit characterization of spatial reuse. We propose link scheduling algorithms based on certain graph models (communication graph, SINR graph) of the network. Our algorithms achieve higher spatial reuse than that of existing algorithms, with only a slight increase in computational complexity. (2) We investigate random access algorithms in wireless networks. We assume that the receiver is capable of power-based capture and propose a splitting algorithm that varies transmission powers of users on the basis of quaternary channel feedback. We model the algorithm dynamics by a Discrete Time Markov Chain and consequently show that its maximum stable throughput is 0.5518. Our algorithm achieves higher maximum stable throughput and significantly lower delay than the First Come First Serve (FCFS) splitting algorithm with uniform transmission power. (3) We consider the problem of flow control in packet networks from an information-theoretic perspective. We derive the maximum entropy of a flow which conforms to traffic constraints imposed by a generalized token bucket regulator (GTBR)...

Critical Points of Correlated Percolation in a Gravitational Link-adding Network Model

Zhu, Chen-Ping; Jia, Long-Tao; Kim, Beom Jun; Wang, Bing-Hong; Stanley, H. E.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/04/2012
Relevância na Pesquisa
26.29%
Motivated by the importance of geometric information in real systems, a new model for long-range correlated percolation in link-adding networks is proposed with the connecting probability decaying with a power-law of the distance on the two-dimensional(2D) plane. By overlapping it with Achlioptas process, it serves as a gravity model which can be tuned to facilitate or inhibit the network percolation in a generic view, cover a broad range of thresholds. Moreover, it yields a set of new scaling relations. In the present work, we develop an approach to determine critical points for them by simulating the temporal evolutions of type-I, type-II and type-III links(chosen from both inter-cluster links, an intra-cluster link compared with an inter-cluster one, and both intra-cluster ones, respectively) and corresponding average lengths. Numerical results have revealed objective competition between fractions, average lengths of three types of links, verified the balance happened at critical points. The variation of decay exponents $a$ or transmission radius $R$ always shifts the temporal pace of the evolution, while the steady average lengths and the fractions of links always keep unchanged just as the values in Achlioptas process. Strategy with maximum gravity can keep steady average length...

The random link approximation for the Euclidean traveling salesman problem

Cerf, N. J.; de Monvel, J. Boutet; Bohigas, O.; Martin, O. C.; Percus, A. G.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.04%
The traveling salesman problem (TSP) consists of finding the length of the shortest closed tour visiting N ``cities''. We consider the Euclidean TSP where the cities are distributed randomly and independently in a d-dimensional unit hypercube. Working with periodic boundary conditions and inspired by a remarkable universality in the kth nearest neighbor distribution, we find for the average optimum tour length = beta_E(d) N^{1-1/d} [1+O(1/N)] with beta_E(2) = 0.7120 +- 0.0002 and beta_E(3) = 0.6979 +- 0.0002. We then derive analytical predictions for these quantities using the random link approximation, where the lengths between cities are taken as independent random variables. From the ``cavity'' equations developed by Krauth, Mezard and Parisi, we calculate the associated random link values beta_RL(d). For d=1,2,3, numerical results show that the random link approximation is a good one, with a discrepancy of less than 2.1% between beta_E(d) and beta_RL(d). For large d, we argue that the approximation is exact up to O(1/d^2) and give a conjecture for beta_E(d), in terms of a power series in 1/d, specifying both leading and subleading coefficients.; Comment: 29 pages, 6 figures; formatting and typos corrected

Comparing Mean Field and Euclidean Matching Problems

Houdayer, J.; de Monvel, J. H. Boutet; Martin, O. C.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.2%
Combinatorial optimization is a fertile testing ground for statistical physics methods developed in the context of disordered systems, allowing one to confront theoretical mean field predictions with actual properties of finite dimensional systems. Our focus here is on minimum matching problems, because they are computationally tractable while both frustrated and disordered. We first study a mean field model taking the link lengths between points to be independent random variables. For this model we find perfect agreement with the results of a replica calculation. Then we study the case where the points to be matched are placed at random in a d-dimensional Euclidean space. Using the mean field model as an approximation to the Euclidean case, we show numerically that the mean field predictions are very accurate even at low dimension, and that the error due to the approximation is O(1/d^2). Furthermore, it is possible to improve upon this approximation by including the effects of Euclidean correlations among k link lengths. Using k=3 (3-link correlations such as the triangle inequality), the resulting errors in the energy density are already less than 0.5% at d>=2. However, we argue that the Euclidean model's 1/d series expansion is beyond all orders in k of the expansion in k-link correlations.; Comment: 11 pages...

Influence of the Measure on Simplicial Quantum Gravity in Four Dimensions

Beirl, W.; Gerstenmayer, E.; Markum, H.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 22/04/1992
Relevância na Pesquisa
26.09%
We investigate the influence of the measure in the path integral for Euclidean quantum gravity in four dimensions within the Regge calculus. The action is bounded without additional terms by fixing the average lattice spacing. We set the length scale by a parameter $\beta$ and consider a scale invariant and a uniform measure. In the low $\beta$ region we observe a phase with negative curvature and a homogeneous distribution of the link lengths independent of the measure. The large $\beta$ region is characterized by inhomogeneous link lengths distributions with spikes and positive curvature depending on the measure.; Comment: 12pgs

The Link Prediction Problem in Bipartite Networks

Kunegis, Jérôme; De Luca, Ernesto W.; Albayrak, Sahin
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 28/06/2010
Relevância na Pesquisa
26.09%
We define and study the link prediction problem in bipartite networks, specializing general link prediction algorithms to the bipartite case. In a graph, a link prediction function of two vertices denotes the similarity or proximity of the vertices. Common link prediction functions for general graphs are defined using paths of length two between two nodes. Since in a bipartite graph adjacency vertices can only be connected by paths of odd lengths, these functions do not apply to bipartite graphs. Instead, a certain class of graph kernels (spectral transformation kernels) can be generalized to bipartite graphs when the positive-semidefinite kernel constraint is relaxed. This generalization is realized by the odd component of the underlying spectral transformation. This construction leads to several new link prediction pseudokernels such as the matrix hyperbolic sine, which we examine for rating graphs, authorship graphs, folksonomies, document--feature networks and other types of bipartite networks.; Comment: 10 pages

Scaling of optimal-path-lengths distribution in complex networks

Kalisk, Tomer; Braunstein, Lidia A.; Buldyrev, Sergey V.; Havlin, Shlomo; Stanley, H. Eugene
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 01/08/2005
Relevância na Pesquisa
26.18%
We study the distribution of optimal path lengths in random graphs with random weights associated with each link (``disorder''). With each link $i$ we associate a weight $\tau_i = \exp(ar_i)$ where $r_i$ is a random number taken from a uniform distribution between 0 and 1, and the parameter $a$ controls the strength of the disorder. We suggest, in analogy with the average length of the optimal path, that the distribution of optimal path lengths has a universal form which is controlled by the expression $\frac{1}{p_c}\frac{\ell_{\infty}}{a}$, where $\ell_{\infty}$ is the optimal path length in strong disorder ($a \to \infty$) and $p_c$ is the percolation threshold. This relation is supported by numerical simulations for Erd\H{o}s-R\'enyi and scale-free graphs. We explain this phenomenon by showing explicitly the transition between strong disorder and weak disorder at different length scales in a single network.

On the Maximum Span of Fixed-Angle Chains

Benbernou, Nadia; O'Rourke, Joseph
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.09%
Soss proved that it is NP-hard to find the maximum 2D span of a fixed-angle polygonal chain: the largest distance achievable between the endpoints in a planar embedding. These fixed-angle chains can serve as models of protein backbones. The corresponding problem in 3D is open. We show that three special cases of particular relevance to the protein model are solvable in polynomial time. When all link lengths and all angles are equal, the maximum 3D span is achieved in a flat configuration and can be computed in constant time. When all angles are equal and the chain is simple (non-self-crossing), the maximum flat span can be found in linear time. In 3D, when all angles are equal to 90 deg (but the link lengths arbitrary), the maximum 3D span is in general nonplanar but can be found in quadratic time.; Comment: 28 pages, 21 figures. Preliminary version appeared in Proc. 18th Canad. Conf. Comput. Geom., pages 93-96, 2006. This paper has been withdrawn by the authors. Lemma 15 as stated is incorrect, and although we believe the main theorems following (Thms. 17 & 18) are true, the proofs relying on Lem.15 are not valid

Measuring the dimension of partially embedded networks

Kondor, Dániel; Mátray, Péter; Csabai, István; Vattay, Gábor
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 28/08/2013
Relevância na Pesquisa
26.04%
Scaling phenomena have been intensively studied during the past decade in the context of complex networks. As part of these works, recently novel methods have appeared to measure the dimension of abstract and spatially embedded networks. In this paper we propose a new dimension measurement method for networks, which does not require global knowledge on the embedding of the nodes, instead it exploits link-wise information (link lengths, link delays or other physical quantities). Our method can be regarded as a generalization of the spectral dimension, that grasps the network's large-scale structure through local observations made by a random walker while traversing the links. We apply the presented method to synthetic and real-world networks, including road maps, the Internet infrastructure and the Gowalla geosocial network. We analyze the theoretically and empirically designated case when the length distribution of the links has the form P(r) ~ 1/r. We show that while previous dimension concepts are not applicable in this case, the new dimension measure still exhibits scaling with two distinct scaling regimes. Our observations suggest that the link length distribution is not sufficient in itself to entirely control the dimensionality of complex networks...