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soleimani-roudi--saman
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Improving Robustness of Deep Networks to Adversarial Perturbations based on Ensemble Structure
, M.Sc. Thesis Sharif University of Technology ; Ghaemmaghami, Shahrokh (Supervisor) ; Marvasti, Farokh (Supervisor) ; Amini, Sajjad (Co-Supervisor)
Abstract
Deep learning is one of the most powerful branches of machine learning, demonstrating exceptional performance in tasks such as image and speech recognition with the help of neural networks. However, deep learning networks are highly vulnerable to small and intentional changes in input data, known as adversarial perturbations. These perturbations, despite having a significant impact on the model's output, are usually imperceptible to humans and can pose serious security challenges in sensitive areas such as autonomous vehicles and medical diagnostics. To counter these perturbations, extensive research has been conducted, most of which has utilized a single deep learning network to create a...
Phase transitions and asymmetry between signal comprehension and production in biological communication
, Article Scientific Reports ; Volume 9, Issue 1 , 2019 ; 20452322 (ISSN) ; Rouhani, S ; Roudi, Y ; Sharif University of Technology
Nature Publishing Group
2019
Abstract
We introduce a model for collective information acquisition from the environment, in a biological population. In this model, individuals can make noisy observations of the environment, and communicate their observation by production and comprehension of signals. As the communication noise decreases, the model shows an order-disorder transition from a disordered phase in which no consensus about the environmental state exists to an ordered phase where the population forms a consensus about the environmental state. The ordered phase itself is composed of an informed consensus, in which the correct belief about the environment prevails, and an uninformed consensus phase, in which consensus on a...
Critical Behavior of Neuronal Systems: an Information Theory Viewpoint
, M.Sc. Thesis Sharif University of Technology ; Moghimi, Saman (Supervisor)
Abstract
Experiments conducted in recent two decades indicated critical behavior in neural activity at different scales. Theoretically occurrences of these critical and power-law behavior can significantly facilitate brain activities correspondent to computation and memory tasks, but attaining the critical point essentially demands externally fine-tuning which has not been established yet. This fine-tuning often lies with placing system at transition point. Recent studies of group showed that a transition from synchronous to asynchronous phase could be achievable by a change in external parameters. At the very transition point, neuronal avalanches statistically demonstrate a power-law behavior which...
Modelling the Physics of Collective Decision Making and Commuication by Signal Exchange
, Ph.D. Dissertation Sharif University of Technology ; Rouhani, Shahin (Supervisor) ; Roudi, Yaser (Co-Supervisor)
Abstract
In many biological populations, information about environmental conditions is acquired collectively, through information sharing among individuals, by exchanging signals. In order to achieve a better understanding of such systems, using methods borrowed from statistical physics, we introduce two mathematical models. First, a collective decision making model, for a community of individuals residing on a communication network, who live in an uncertain environment, in which individuals try to collectively find the environmental state. In the second model, a collective movement model, we consider a population of collectively moving individuals, in which individuals try to collectively find and...
Sandpiles and Surface Growth
, M.Sc. Thesis Sharif University of Technology ; Moghimi Araghi, Saman (Supervisor)
Abstract
We study the Abelian Sandpile Model and its relation with surface growth. ese two models are related through their field theories and equations of motion. It has been shown that the different features of different sandpile models can be expressed in terms of the noise term in the surface growth equation. A mapping between the simplest sandpile model, the BTW model, and a surface growth has already been introduced. is surface growth has not been studied in details so far. In this thesis we study different features of this surface growth corresponding to the BTW model, continuous sandpile model and also massive abelian sandpile model. We also consider different boundary conditions
The Abelian Sand-pile Model (ASM) and Generalization to the Continuous State
, M.Sc. Thesis Sharif University of Technology ; Moghimi Araghi, Saman (Supervisor)
Abstract
The four-page article by Bak, Tang and Wiesenfeld in 1987 was a beginning to a new wave of physicists’ efforts to explain and describe the concept of complexity; a not-so-well-defined concept that resists against the reductionist tools and methods of physics. The Self-organized Criticality theory presented in that article via a simple model, known as sandpile model, was first of all an effort to explain the numerous occurrence of power law distribution in nature. SOC was introduced to tell us why so many natural phenomena like Earthquakes, landslides, forest fires, extinction and other seemingly non-related catastrophic events, more or less obey the scale-less power law distribution; A...
Transition from Abelian Sandpile Model to Manna Model
, M.Sc. Thesis Sharif University of Technology ; Moghimi-Araghi, Saman (Supervisor)
Abstract
In this research, we want to address the question of universality classes in BTW and Manna sandpile models. So far, number of works has been devoted to this issue but the the answer remained unsolved. We will try another approach to study this question by perturbing the original models. To this end, we introduce three models that have evolution rules between BTW model and Manna model. By simulating this models, we observe that in the presence of perturbation, the probability dis- tribution has two regimes of behaviour which are separated by a new characteristic scale. The regime of small avalanches is described by the exponent of BTW model and the regime of large avalanches by the exponent...
Generalized Growth Models
, M.Sc. Thesis Sharif University of Technology ; Moghimi-Araghi, Saman (Supervisor)
Abstract
Edwards-Wilkinson’s equation can be achieved from a Hamiltonian. When we have the Hamiltonian for the system, there are common approaches that makes it out of critical. In other words,the ”mass” should be added to the system. In this study we have tried to simulate and solve analytically these models that are involved mass term. We try to onstruct these mass terms in a way that have a minimum impact on the system and we study the quantities that characterize the out of critical behaviors
Percolation on Small World Networks
, M.Sc. Thesis Sharif University of Technology ; Moghimi Araghi, Saman (Supervisor)
Abstract
Percolation is a phenomenon that can be found in many physical problems. Additionally, as a statistical model, it has a very rich physics, since many fundamental concepts in the context of critical phenomena and complex systems-such as phase transition, scaling laws etc can be found in the model. Percolation phenomenon can be defined on different lattices. In this thesis we study percolation on small-world networks. In small-world networks, in addition to local bonds that connects the neighbouring sites, there exist some long-ranged bonds that connect cites far from each other. Social networks, some networks of internet or the gene networks are examples of such networks. Therefore, to study...
Chaos in Sandpile Models With and Without Bulk Dissipation
, M.Sc. Thesis Sharif University of Technology ; Moghimi-Araghi, Saman (Supervisor)
Abstract
A complte set of characteristic parameters of the sandpile models is still unknown. We have studied the existence of ”weak chaos” critical exponent in different sandpile models and we have shown that it is a characteristic exponent of deterministic models. We have shown that BTW and Zhang models do not belong to the same universality class (contrary to Zhang’s previous conjecture and contrary to Ben-Hur & Biham’s results.) Also we have shown that directed models, specificly Ramaswamy-Dhar’s directed model form a different universality class. ”Weak chaos” exponent in also studied in massive models and we have shown that by increase of dissipation, the exponent decreases rapidly to an...
Simulation of the Self-organized Critical Models on the
Human’s Brain Network
,
M.Sc. Thesis
Sharif University of Technology
;
Moghimi Araghi, Saman
(Supervisor)
Abstract
Self-organized critical phenomena are interesting phenomena which are ubiquitous in nature. Examples include mountain ranges , coastlines and also activities in the hu-man's brain. In these processes, without fine-tuning of any external parameter such as the temperature, the system exhibits critical behavior. In other words, the dynamics of the system, drives it towards an state in which long range correlations in space and scaling behaviors can be seen.The first successful model which could characterize such systems was BTW model, introduced by Bak , Tang and Wiesenfeld in 1987. This model, later named Abelian sandpile model, was very simple and because of this simplicity, a large amount of...
Fluctuations in the order of System Size in the Avalanche-Size Distribution of Sandpiles Model
, M.Sc. Thesis Sharif University of Technology ; Moghimi Araghi, Saman (Supervisor)
Abstract
Since the concept of Self-Organized Criticality was introduced in terms of BTW Sandpiles model, its major features have been known as broad power law distributions without any tuning parameters. In some selforganized critical systems like brain and neural networks, some evidences and experiments show a periodic or non-power law distribution of avalanches in addition to the power-law distributions of avalanches. In this thesis we try to observe the same phenomenon in the well-known SOC models, namely the BTW and Manna sandpile models. We have considered small lattice sizes with periodic boundary conditions and a small amount of dissipation. Within such conditions we observe a periodic-like...
Burridge-Knopoff Model with Nonuniform Parameters
, M.Sc. Thesis Sharif University of Technology ; Moghimi-Araghi, Saman (Supervisor)
Abstract
Power law behavior of earthquakes has been a matter of interest for many scientists. One on these power laws known as Gutenberg-Richter law describes the magnitude distribution of earthquakes. The Burridge-Knopoff model of faults, produces the same power law distribution of events as the Gutenberg-Richter law for earthquakes. Olami, Feder and Christensen in 1992, introduced a 2-D, continues sand pile model Known as OFC that displays self-organized-criticality. They claimed that this model is equivalent to Burridge-Knopoff model. It means that criticality is the origin of power law behavior of the Burridge-Knopoff model. Nevertheless, there are some evidence against criticality in the...
Mullins-Herring Equation with Lateral Growth
, M.Sc. Thesis Sharif University of Technology ; Moghimi Araghi, Saman (Supervisor)
Abstract
Surface growth have been one of the most interesting topics of research in non-equilibrium Statistical physics, due to their relevance in studying industrial growth processes. Many models such as Edwards-Wilkinson and KPZ have been proposed to study these systems where by incorporating renormalization group, numerical integration and computer simulations we can derive their critical exponents. In general, a thermal noise is implemented in these models, however, other types can be used as well. In particular for the case of Edwards-Wilkinson, it has been shown that a multiplicative noise changes the universality class of the model. In this thesis we want to investigate the effects of...
The Role of Simplified Models for Neurons in the Emergence of Collective Behaviors in Neuronal Populations
, M.Sc. Thesis Sharif University of Technology ; Moghimi Araghi, Saman (Supervisor)
Abstract
The brain, as a complex system with various components working in concert, plays a fundamental role in many cognitive processes and human perception of the surrounding environment. Perception, in many cases, can differ from reality due to evolutionary processes, natural selection, or even flaws in any of the parameters within this complex intelligent system. One of the observed phenomena in perception is the heightened visibility of edges, often depicted with Mach bands. To describe this phenomenon, primarily, rate models are used, especially in regions where a linear approximation is suitable for neuronal responses. However, the response of neuronal populations in a range of external...
Investigating Scaling Behaviour of Avalanches in Sandpiles on Small-World Networks Using the Renormalization Approach
,
M.Sc. Thesis
Sharif University of Technology
;
Moghimi Araghi, Saman
(Supervisor)
Abstract
The Manna sandpile model is a significant and widely-used model in the study of self-organized criticality. Various avalanche-related quantities, such as area, size, duration, and others, exhibit power-law distributions with finite-size effects. It has been demonstrated that this model exhibits simple monofractal behavior on both regular lattices and random networks, and the finite-size scaling (FSS) hypothesis holds for the avalanche distribution functions. On the other hand, it has been observed that a wide range of natural and human-made networks are small-world networks. Consequently, studying the Manna sandpile model on such networks and understanding its features and behavior can...
Studying Continuum Percolation Models using Gap Scaling Approach
, M.Sc. Thesis Sharif University of Technology ; Moghimi Araghi, Saman (Supervisor)
Abstract
Percolation is a fundamental topic in statistical physics with applications in modeling natural phenomena such as the spread of diseases and forest fires. It describes a geometric phase transition in which a system evolves from small-scale connectivity to extensive connectedness. At the percolation threshold, the system undergoes a continuous phase transition, exhibiting critical behavior. A recent study introduced a novel method for determining the percolation threshold by measuring the maximum change in the size of the largest cluster during the gradual increase of a control parameter. This maximum change, known as the ”gap,” signals the occurrence of the phase transition. The study also...
Production planning problem with pricing under random yield: CVAR criterion [electronic resource]
, Article Journal of Systems Science and Systems Engineering ; 2014 ; Eshghi, Kourosh ; Modarres Yazdi, Mohammad ; Bahramgiri, Mohsen ; Sharif University of Technology
Abstract
In this paper, we address a basic production planning problem with price dependent demand and stochastic yield of production. We use price and target quantity as decision variables to lower the risk of low yield. The value of risk control becomes more important especially for products with short life cycle. This is because, the profit implications of low yield might be unbearable in the short run. We apply Conditional Value at Risk (CVaR) to model the risk. CVaR measure is a coherent risk measure and thereby having nice conceptual and mathematical underpinnings. It is also widely used in practice. We consider the problem under general demand function and general distribution function of...
Throughput analysis of a cognitive multi-hop IEEE 802.11-based WLAN overlaid on a cellular network
, Article 2011 International Conference on Software, Telecommunications and Computer Networks, SoftCOM 2011, 15 September 2011 through 17 September 2011, Split, Hvar, Dubrovnik ; 2011 , Pages 307-311 ; 9789532900262 (ISBN) ; Ashtiani, F ; Sharif University of Technology
2011
Abstract
In this paper, we focus on a cognitive network scenario, comprised of a multi-hop IEEE 802.11-based WLAN overlaid on a cellular network. In this scenario, the wireless nodes in a WLAN, i.e., cognitive nodes, use the downlink frequency channels of the cellular network opportunistically, such that at each packet transmission all idle channels have been exploited. In order to evaluate the capability of the considered cognitive network scenario, we propose an open queueing network model such that different packet transmission phases are mapped onto different queueing nodes. We also map different types of collisions, i.e., collision on control signals as well as collision on data transmission due...
Synergistic strengthening by severe plastic deformation and post-heat treatment of a low-carbon steel
, Article Steel Research International ; Volume 89, Issue 6 , 2018 ; 16113683 (ISSN) ; Kazeminezhad, M ; Sharif University of Technology
Wiley-VCH Verlag
2018
Abstract
Low-carbon steel sheets are severely plastic deformed to strains of up to ≈3.48 and subsequently heat treated by conventional annealing followed by water-quenching. Four temperatures are chosen for the annealing below and over the Ac1 and Ac3 transformation lines. The effects of post-deformation heat treatment are investigated by evaluating the microstructure and mechanical properties, including strength, ductility, work hardening capability, and hardness. A maximum increase of 86% in the strength is obtained through intercritical annealing and quenching of the samples subjected to strain of 1.16. It is interesting that both the elongation and ultimate tensile strength values are higher...