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Ali Pinar

Bio: Ali Pinar is an academic researcher from Sandia National Laboratories. The author has contributed to research in topics: North Anatolian Fault & Aftershock. The author has an hindex of 42, co-authored 247 publications receiving 5642 citations. Previous affiliations of Ali Pinar include Kandilli Observatory and Earthquake Research Institute & Boğaziçi University.


Papers
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Journal ArticleDOI
TL;DR: The Block Two-Level Erdős-Rényi (BTER) model is proposed, and it is demonstrated that it accurately captures the observable properties of many real-world social networks.
Abstract: Community structure plays a significant role in the analysis of social networks and similar graphs, yet this structure is little understood and not well captured by most models. We formally define a community to be a subgraph that is internally highly connected and has no deeper substructure. We use tools of combinatorics to show that any such community must contain a dense Erd\ifmmode \mbox{\H{o}}\else \H{o}\fi{}s-R\'enyi (ER) subgraph. Based on mathematical arguments, we hypothesize that any graph with a heavy-tailed degree distribution and community structure must contain a scale-free collection of dense ER subgraphs. These theoretical observations corroborate well with empirical evidence. From this, we propose the Block Two-Level Erd\ifmmode \mbox{\H{o}}\else \H{o}\fi{}s-R\'enyi (BTER) model, and demonstrate that it accurately captures the observable properties of many real-world social networks.

292 citations

Proceedings ArticleDOI
01 Jan 1999
TL;DR: Alternative data structures are proposed, along with reordering algorithms to increase effectiveness of these data structures, to reduce the number of memory indirections inparse matrix-vector multiplication (SpMxV).
Abstract: Sparse matrix-vector multiplication (SpMxV) is one of the most important computational kernels in scientific computing. It often suffers from poor cache utilization and extra load operations because of memory indirections used to exploit sparsity. We propose alternative data structures, along with reordering algorithms to increase effectiveness of these data structures, to reduce the number of memory indirections. Toledo proposed handling the 1x2 blocks of a matrix separately, doing only one indirection for each block. We propose packing all contiguous nonzeros into a block to reduce the number of memory indirections further. This reduces memory indirections per block to one for the cost of an extra array in storage and a loop during SpMxV. We also propose an algorithm to permute the nonzeros of the matrix into contiguous locations. We state this problem as the traveling salesperson problem and use associated heuristics. Experiments verify the effectiveness of our techniques.

231 citations

Journal ArticleDOI
TL;DR: In this article, the average shear wave velocity for the top 30 m of soil is referred to as V S 30, which is an important parameter for evaluating the dynamic behaviour of soil in the shallow subsurface.
Abstract: SUMMARY The Dinar earthquake (M S = 6.1) of 1995 October 1 killed 90 people and destroyed more than 4000 buildings. Despite the moderate size of the earthquake, the level of damage was extremely high, which led to many studies that were carried out in the region. The majority of these studies concluded that the main reasons for the damage were the construction errors and the poor soil conditions. However, at that time no appropriate soil condition map based on extended, high density measurements was available. Shear wave velocity is an important parameter for evaluating the dynamic behaviour of soil in the shallow subsurface. Thus site characterization in calculating seismic hazards is usually based on the near surface shear wave velocity values. The average shear wave velocity for the top 30 m of soil is referred to as V S 30 . For earthquake engineering design purposes, both the Uniform Building Code (UBC) and Eurocode 8 (EC8) codes use V S 30 to classify sites according to the soil type. The V s 30 values calculated by using multichannel analysis of surface waves (MASW) were used to create a new soil classification map of the Dinar region. Surface seismic measurements were carried out at 50 locations mostly in Dinar city and its surroundings. The dispersion data of the recorded Rayleigh waves were inverted using a Genetic Algorithm (GA) method to obtain shear wave velocity profiles of the investigated sites. Thus the derived V s 30 map of the Dinar region was transformed to the UBC and EC8 standards. Soil classification results show that most parts of the region, located in alluvial basin, have low shear wave velocity values. These values are within the range of 160–240 m s −1 and thus fall into the S D and S E categories according to the UBC and the C and D categories according to EC8. Within the region, some parts located on the hill zone and the transition zone have better soil conditions [corresponding to S C (UBC) and B (EC8) categories] and have comparatively high shear wave velocities in the range of 500–740 m s −1 and 350–450 m s −1 , respectively. V S 30 and soil classification maps were compared with the damage distribution associated with the earthquake. In possession of a detailed shear wave velocity map of Dinar City, in general, the results show that there is a correlation between the V S 30 values and the damage distribution of the region. In addition to the low V S 30 values, the likely causes of the damage were investigated, and it is observed that one of the major factors for high levels of damage is 3-D variations of geological structures.

156 citations

Journal ArticleDOI
TL;DR: It is proposed that the proposed Block Two-Level Erdss-Renyi (BTER) model can be used as a graph generator for benchmarking purposes and provide idealized degree distributions and clustering coefficient profiles that can be tuned for user specifications.
Abstract: Network data is ubiquitous and growing, yet we lack realistic generative network models that can be calibrated to match real-world data. The recently proposed Block Two-Level Erdss-Renyi (BTER) model can be tuned to capture two fundamental properties: degree distribution and clustering coefficients. The latter is particularly important for reproducing graphs with community structure, such as social networks. In this paper, we compare BTER to other scalable models and show that it gives a better fit to real data. We provide a scalable implementation that requires only O(d_max) storage where d_max is the maximum number of neighbors for a single node. The generator is trivially parallelizable, and we show results for a Hadoop MapReduce implementation for a modeling a real-world web graph with over 4.6 billion edges. We propose that the BTER model can be used as a graph generator for benchmarking purposes and provide idealized degree distributions and clustering coefficient profiles that can be tuned for user specifications.

147 citations

Journal ArticleDOI
TL;DR: To represent the nonzero structure of a matrix, bipartite graph and hypergraph models that reduce the permutation problem to those of graph partitioning by vertex separator andhypergraph partitioning, respectively are proposed.
Abstract: We investigate the problem of permuting a sparse rectangular matrix into block-diagonal form. Block-diagonal form of a matrix grants an inherent parallelism for solving the deriving problem, as recently investigated in the context of mathematical programming, LU factorization, and QR factorization. To represent the nonzero structure of a matrix, we propose bipartite graph and hypergraph models that reduce the permutation problem to those of graph partitioning by vertex separator and hypergraph partitioning, respectively. Our experiments on a wide range of matrices, using the state-of-the-art graph and hypergraph partitioning tools MeTiS and PaToH\@, revealed that the proposed methods yield very effective solutions both in terms of solution quality and runtime.

145 citations


Cited by
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[...]

08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

33,785 citations

01 May 1993
TL;DR: Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems.
Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

29,323 citations

Proceedings ArticleDOI
22 Jan 2006
TL;DR: Some of the major results in random graphs and some of the more challenging open problems are reviewed, including those related to the WWW.
Abstract: We will review some of the major results in random graphs and some of the more challenging open problems. We will cover algorithmic and structural questions. We will touch on newer models, including those related to the WWW.

7,116 citations

Journal ArticleDOI
TL;DR: In most natural and engineered systems, a set of entities interact with each other in complicated patterns that can encompass multiple types of relationships, change in time, and include other types of complications.
Abstract: In most natural and engineered systems, a set of entities interact with each other in complicated patterns that can encompass multiple types of relationships, change in time, and include other types of complications Such systems include multiple subsystems and layers of connectivity, and it is important to take such "multilayer" features into account to try to improve our understanding of complex systems Consequently, it is necessary to generalize "traditional" network theory by developing (and validating) a framework and associated tools to study multilayer systems in a comprehensive fashion The origins of such efforts date back several decades and arose in multiple disciplines, and now the study of multilayer networks has become one of the most important directions in network science In this paper, we discuss the history of multilayer networks (and related concepts) and review the exploding body of work on such networks To unify the disparate terminology in the large body of recent work, we discuss a general framework for multilayer networks, construct a dictionary of terminology to relate the numerous existing concepts to each other, and provide a thorough discussion that compares, contrasts, and translates between related notions such as multilayer networks, multiplex networks, interdependent networks, networks of networks, and many others We also survey and discuss existing data sets that can be represented as multilayer networks We review attempts to generalize single-layer-network diagnostics to multilayer networks We also discuss the rapidly expanding research on multilayer-network models and notions like community structure, connected components, tensor decompositions, and various types of dynamical processes on multilayer networks We conclude with a summary and an outlook

1,934 citations

Journal ArticleDOI
TL;DR: It is shown that the full set of hydromagnetic equations admit five more integrals, besides the energy integral, if dissipative processes are absent, which made it possible to formulate a variational principle for the force-free magnetic fields.
Abstract: where A represents the magnetic vector potential, is an integral of the hydromagnetic equations. This -integral made it possible to formulate a variational principle for the force-free magnetic fields. The integral expresses the fact that motions cannot transform a given field in an entirely arbitrary different field, if the conductivity of the medium isconsidered infinite. In this paper we shall show that the full set of hydromagnetic equations admit five more integrals, besides the energy integral, if dissipative processes are absent. These integrals, as we shall presently verify, are I2 =fbHvdV, (2)

1,858 citations