Sharif Digital Repository

Over the past few years there has been a rapidly growing interest in analysis, design and optimization of various types of collective behaviors in networked dynamic systems. Collective phenomena (such as flocking, schooling, rendezvous, synchronization, and agreement) have been studied in a diverse set of disciplines. In many applications involving multi-agent systems, groups of agents are required to agree on certain quantities of interest; in other words, it is important to develop information consensus protocols for networks of dynamic agents. There are many practical situations where it is desirable or even required to achieve stable convergence in finite-time domain. In this work, a... 

Cooperative system is a system in which agents jointly try to reach a common goal. In order to achieve the desirable goal, a cooperative system needs some conditions such as agents moving as a flock and avoiding to conflict with each other and obstacles. Collision avoidance is one of the most important and notable factor in cooperative systems. This issue has been addressed by many researchers and different solutions have been proposed so far. In this thesis, existing methods and algorithms are studied. Among these algorithms two well-known ones have been investigated having considered their advantages and disadvantages in more detail. Some improvements on these two algorithms are proposed... 

In this thesis, the individuals’ decision process to achieve consensus in complex networks is modeled. A set of experiments with volunteered participants were run, in which the person is a node in a network with 35 other nodes who behave based on a prescribed model. Each participant is tested twice, once with and once without reward. Performance of individuals in the two experiment types and the effect of structural properties of the underlying network on the results are studied. It is observed that increase in the characteristic path length, diameter, clustering coefficient and local efficiency of the network adversely affects consensus.On the other hand, global efficiency and closeness... 

In complex human social activities, practical problems involve more prominent uncertainty, and deterministic approaches of classical methods become powerless. In this research, a new method for human group decision making is presented by using heterogeneous incomplete uncertain preference relations. The uncertain multiplicative preference relations, uncertain fuzzy preference relations, uncertain linguistic preference relations, intuitionistic fuzzy preference relations and Interval
preference sequence can be included in this method.Our new method consists of nine steps. In the first step, decision-makers preferences in the form of heterogeneous comparison matrixes are driven.In the... 

To update Blockchains, a consensus protocol must be used to verify the integrity of registered transactions by users. Bitcoin, as the largest Blockchain network, uses a Proof-of-work mechanism to reach consensus and rewards users for doing so. This will lead to the use of costly hardware to execute heavy processing commands with a lot of energy loss. Different groups have since proposed different mechanisms for reaching consensus without costly processing. The idea we present here is that we first define a general chance variable for each miner, and the miner has to wait a certain amount of time accordingly and then be able to generate the block. Chance variables are public, so other members... 

Designing a scalable protocol for the blockchain will reduce transaction fees and increase the use of cryptocurrencies for financial transactions. Increasing scalability means that if the investment of nodes increases or the number of network nodes increases, the security of the protocol and the decentralization of the network will be maintained and its throughput will increase. In this thesis, with the aim of increasing scalability, the scaling methods of layer one and layer two are examined and compared, and it is shown that sharding is a better method to achieve scalability. In sharding, nodes are divided into several shards (clusters) and each shard manages its own blockchain. The... 

Blockchain systems are considered a subclass of distributed systems and have drawn a lot of attention from academia in recent years. The deployment of these systems in various contexts has been the focus of extensive studies. Because of their design, which places a strong focus on security and decentralization, blockchain systems have some challenges. The most prominent challenge is the one that involves scalability. A blockchain's scalability refers to its capacity to achieve a target throughput and solid performance in confirming transactions with an ever-increasing workload or number of nodes in the network. A solid performance suggests that the blockchain system can sustain its...