arXiv:cond-mat/0107419v3 [cond-mat.stat-mech] 6 Feb 2002Deterministic Scale-Free Networks Albert-L´aszl´o Barab´asia, Erzs´ebet Ravasza, Tam´as Vicsekb aDepartment of Physics, University of Notre Dame, Notre Dame, IN 46556-5670, USA bDepartment of Biological Physics, E¨otv¨os University, P´azm´any P´eter S´etany 1-A, Budapest, Hungary, H-1117 Abstract Scale-free networks are abundant in nature, describing such diverse systems as the world wide web, the web of human sexual contacts, or the chemical network of a cell. All models used to generate a scale-free topology are stochastic, that is they create networks in which the nodes appear to be randomly connected to each other. Here we propose a simple model that generates scale-free networks in a deterministic fashion. We solve exactly the model, showing that the tail of the degree distribution follows a power law. Key words:Disordered Systems; Networks; Scale-free networks; Scaling PACS: Networks are ubiquitous in nature and society [1–3], describing various com- plex systems, such as the society, a network of individuals linked by various social links [4–7]; the Internet, a network of routers connected by various physical connections [8,9]; the world wide web, a virtual web of documents connected by uniform resource connectors [10] or the cell, a network of sub- strates connected by chemical reactions [11,12]. Despite their diversity, most networks appearing in nature follow universal organizing principles. In partic- ular, it has been found that many networks of scientific interest are scale-free [13,14], that is, the probability that a randomly selected node has exactlyk links decays as a power law, followingP(k)∼k−γ, whereγis the degree exponent. The list of documented scale-free networks now include the world wide web [10,15], the Internet [8], the cell [11,12], the web of human sexual contacts [16], the language [17], or the web of actors in Hollywood [18], most of which appear to have degree exponents between two and three. The high interest in understanding the topology of complex networks has resulted in the development of a considerable number ofnetwork models Preprint submitted to Elsevier Preprint22 October 2018
[13,14,19–27]. Most of these are based on two mechanisms: incremental growth and preferential attachment [13,14]. Incremental growth captures the fact that networks are assembled through the addition of new nodes to the system, while preferential attachment encodes the hypothesis that new nodes connect with higher probability to more connected nodes. Both of these mechanisms have been supported by extensive empirical measurements [6,28,29], indicating that they are simultaneously present in many systems with scale-free network topology. Stochasticity is a common feature of all network models that generate scale- free topologies. That is, new nodes connect using a probabilistic rule to the nodes already present in the system. This randomness present in the models, while in line with the major features of networks seen in nature, makes it harder to gain a visual understanding of what makes them scale-free, and how do different nodes relate to each other. It would therefore be of major theoretical interest to construct models that lead to scale-free networks in a deterministic fashion. Here we present such a simple model, generating a deterministic scale-free network using a hierarchical construction. 1Model description The construction of the model, that follows a hierarchical rule commonly used in deterministic fractals [30,31], is shown in Figure 1. The network is built in an iterative fashion, each iteration repeating and reusing the elements generated in the previous steps as follows: Step 0:We start from a single node, that we designate as therootof the graph. Step 1:We add two more nodes, and connect each of them to the root. Step 2:We add two units of three nodes, each unit identical to the network created in the previous iteration (step 1), and we connect each of thebottom nodes (see Figure 1) of these two units to the root. That is, the root will gain four more new links. Step 3:We add two units of nine nodes each, identical to the units generated in the previous iteration, and connect all eight bottom nodes of the two new units to the root. These rules can be easily generalized. Indeed, stepnwould involve the follow- ing operation: Stepn:Add two units of 3n−1nodes each, identical to the network created in the previous iteration (stepn−1), and connect each of the 2nbottom nodes 2
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