Papers by Mohammad Ghodsi
We study the optimal pricing for revenue maximization over social networks in the presence of pos... more We study the optimal pricing for revenue maximization over social networks in the presence of positive network externalities. In our model, the value of a digital good for a buyer is a function of the set of buyers who have already bought the item. In this setting, a decision to buy an item depends on its price and also on the set of other buyers that have already owned that item.
Web pages change frequently and thus crawlers have to download them often. Various policies have ... more Web pages change frequently and thus crawlers have to download them often. Various policies have been proposed for refreshing local copies of web pages. In this paper, we introduce a new sampling method that excels over other change detection methods in experiment. Change Frequency (CF) is a method that predicts the change frequency of the pages and, in the long run, achieves an optimal efficiency in comparison with the sampling method.
Computing the visibility polygon, VP, of a point in a polygonal scene, is a classical problem tha... more Computing the visibility polygon, VP, of a point in a polygonal scene, is a classical problem that has been studied extensively. In this paper, we consider the problem of computing VP for any query point efficiently, with some additional preprocessing phase. The scene consists of a set of obstacles, of total complexity O (n). We show for a query point q, VP (q) can be computed in logarithmic time using O (n 4) space and O (n 4 logn) preprocessing time.
In this paper, performance modeling of synchronization delays is considered in a distributed prog... more In this paper, performance modeling of synchronization delays is considered in a distributed program which consists of a number of processes that interact via message passing only. A class of timed Petri nets called Stochastic Decision Free Petri Nets (SDFPNs) is used to model such distributed programs with deterministic control flow. An exact solution technique is proposed for this model which does not follow the usual approach of reachability analysis for Petri nets and solving global balance equations for a Markovian system.
We study a classical problem in communication and wireless networks called Finding White Space Re... more We study a classical problem in communication and wireless networks called Finding White Space Regions. In this problem, we are given a set of antennas (points) some of which are noisy (black) and the rest are working fine (white). The goal is to find a set of convex hulls with maximum total area that cover all white points and exclude all black points. In other words, these convex hulls make it safe for white antennas to communicate with each other without any interference with black antennas.
Abstract This paper considers scheduling tasks while minimizing the power consumption of one or m... more Abstract This paper considers scheduling tasks while minimizing the power consumption of one or more processors, each of which can go to sleep at a fixed cost α. There are two natural versions of this problem, both considered extensively in recent work: minimize the total power consumption (including computation time), or minimize the number of" gaps" in execution. For both versions in a multiprocessor system, we develop a polynomial-time algorithm based on sophisticated dynamic programming.
Polar Diagram [4] is a new locus approach for problems processing angles. The solution to many im... more Polar Diagram [4] is a new locus approach for problems processing angles. The solution to many important problems in Computational Geometry requires some kind of angle processing of the data input. Using the Polar Diagrma as preprocessing, exhaustive searches to find those sites with smallest angle become unnecessary. In this paper, we use the notion of kinetic data structure [1][2] to model the dynamic case of polar diagram, ie we maintain the polar diagram of a set of continuously moving objects in the scene.
Abstract Redundant and hybrid-redundant number representations are used extensively to speed up a... more Abstract Redundant and hybrid-redundant number representations are used extensively to speed up arithmetic operations within general-purpose and special-purpose digital systems, with the latter (containing both redundant and nonredundant digits) offering cost advantages over fully redundant systems. We use weighted bit-set (WBS) encoding as a paradigm for uniform treatment of five previously studied variants of hybrid-redundant systems.
Determining whether two segments s and t in a planar polygonal scene weakly see each other is a c... more Determining whether two segments s and t in a planar polygonal scene weakly see each other is a classical problem in computational geometry. In this problem we seek for a segment connecting two points of s and t without intersecting edges of the scene. In planar polygonal scenes, this problem is 3sum-hard and its time complexity is Ω (n 2) where n is the complexity of the scene. This problem can be defined in the same manner when s and t are any kind of objects in the plane.
We present three streaming algorithms that (ε, δ)–approximate the number of triangles in graphs. ... more We present three streaming algorithms that (ε, δ)–approximate the number of triangles in graphs. Similar to the previous algorithms [3], the space usage of presented algorithms are inversely proportional to the number of triangles while, for some families of graphs, the space usage is improved. We also prove a lower bound, based on the number of triangles, which indicates that our first algorithm behaves almost optimally on graphs with constant degrees.
Pipelined operator tree (POT) scheduling is an important problem in the area of parallel query op... more Pipelined operator tree (POT) scheduling is an important problem in the area of parallel query optimization. A POT is a tree with nodes representing query operators that can run in parallel and edges representing communication between adjacent operators that is handled by sending long streams of data in a parallel-pipelined fashion. The problem is to find a schedule for the POT that minimizes the total response time.
Abstract In this paper, we study the problem of finding the shortest path in a polygonal domain i... more Abstract In this paper, we study the problem of finding the shortest path in a polygonal domain in which the path should meet (touch or cross) a simple polygon in the domain. Our method uses the continuous Dijkstra paradigm and reflected wavefronts to solve the problem in worst-case optimal time O (nlog n).
Abstract One of the challenges of P2P systems is to perform load balancing efficiently. A distrib... more Abstract One of the challenges of P2P systems is to perform load balancing efficiently. A distributed hash table (DHT) abstraction, heterogeneous nodes, and non uniform distribution of objects cause load imbalance in structured P2P overlay networks. Several solutions are suggested to solve this problem but they have some restrictions. They assume the homogeneous capabilities of nodes, unawareness of the link latency during transferring load and imposing logical structures to collect and reassign load.
Many studies have been focused on cytogenetic and molecular genetic defects in brain tumors; ther... more Many studies have been focused on cytogenetic and molecular genetic defects in brain tumors; therefore the role of ATM as a tumor suppressor gene in these tumors is poorly considered. In this study mutation analysis of exon 19 and 39 of ATM gene and P53 accumulation were investigated by PCR-SSCP, sequencing, and flow cytometry . Four polymorphisms including D1853N, IVS 38-8 T→C, F858L, P872T were reported for the first time in brain tumors other than medulloblastoma. Expression of P53 could be detected in more than 10% of cells in patients affected with meningioma and 4.08% and 3.46% of cells inastrocytoma and chordoma respectively. The present findings could confirm the importance of ATM gene alterations in tumor genesis of brain tumors and further investigation is essential.
Abstract Lazy bureaucrat scheduling is a new class of scheduling problems that was introduced in ... more Abstract Lazy bureaucrat scheduling is a new class of scheduling problems that was introduced in 1]. In this class of scheduling problems, there is one employee (or more) who should perform the assigned jobs; the objective of the employee is to minimize the amount of work he does and be as ine cient as possible (he is\ Lazy"). He is subject to a constraint that he should be busy when there is work to be done. In this paper, we rst brie y de ne the\ lazy bureaucrat scheduling" model as it is introduced in 1] and present some old results.
Abstract We consider a market in which two competing sellers offer two similar products on a soci... more Abstract We consider a market in which two competing sellers offer two similar products on a social network. In this market, each agent chooses iteratively between the products based on her neighbors reactions and prices. This introduces two games; one between the agents and one between the sellers. We show the first game is a full potential game and provide an algorithm to compute its convergence point. We also study various properties of the second game such as its equilibrium points and convergence.
Abstract We study permutation betting markets, introduced by Chen et al.(Proceedings of the ACM C... more Abstract We study permutation betting markets, introduced by Chen et al.(Proceedings of the ACM Conference on Electronic Commerce, 2007). For these markets, we consider subset bettings in which each trader can bet on a subset of candidates ending up in a subset of positions.
Abstract This paper presents a polynomial-time approximation algorithm for labeling some of the p... more Abstract This paper presents a polynomial-time approximation algorithm for labeling some of the points in a given set of weighted points with arbitrary sliding labels of bounded height and width to maximize the weight of the labeled points. We generate a finite set of labels which contains a labeling with weight of 1− ϵ times of the optimum labeling, for arbitrary fixed ϵ> 0, then we use this set as the input for the fixed position problem.
Abstract The visibility graph is a fundamental geometric structure which is useful in many applic... more Abstract The visibility graph is a fundamental geometric structure which is useful in many applications, including illumination and rendering, motion planning, pattern recognition, and sensor networks. While the concept of visibility graph is widely studied for 2D scenes, there is not any acceptable equivalence of visibility graph for 3D space. In this paper we explain some reason for this absence. Then we try to find a new way to define geometric structure in 3D space.
Background: One of the most important complications during pregnancy is hypertension which is res... more Background: One of the most important complications during pregnancy is hypertension which is responsible for a widespread range of harms in mothers and fetuses. This investigation was designed to study the prevalence of hypertensive disorders during pregnancy in Shiraz, Iran. Methods: This is a descriptive cross-sectional study, and comprised all pregnant women referred to Hafez and Zeinnabieh hospitals affiliated to Shiraz University of Medical sciences.
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Papers by Mohammad Ghodsi