Abstract
One of the most important concepts in network analysis is to understand the structure of a given graph with respect to a set of suitably randomized graphs, a so-called random graph model. Structures which are found to be significantly different from those expected in the random graph model require a new random graph model which exemplifies how the structure might emerge in the complex network. In this chapter the most common random graph models are introduced: the classic Erdős-Rényi model, the small-world model by Watts and Strogatz, and the preferential attachment model by Barabási and Albert.
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Notes
- 1.
In the following, if not stated otherwise, the word “graph” should always be understood as “simple graph”, i.e., not containing self-loops or multiple edges.
- 2.
In computer science, an instance of a mathematical model is a concrete entity chosen according to its probability.
- 3.
Chung differentiates between off-line and on-line or generative models. The first ones are those based on a fixed number of vertices, in the second type of model, at each time step edges and vertices might be added or deleted [13, p. 17].
- 4.
Bollobás and Riordan write that actually this model was first published and analyzed by Gilbert in [19]. A similar model was described even earlier by Solomonoff and Rapoport: in it, every node is assigned some number k of outgoing edges each of which is connected to every other node with the same probability [39]. This can be seen as a k-out-regular random graph model.
- 5.
Square brackets denote an interval of real numbers, i.e., [0, 1] means that p is any number between 0 and 1.
- 6.
- 7.
Asymptotic means that the diameter of the graph will approach this value closer and closer for increasing n.
- 8.
Note that the double average is intended: it is once averaged over all pairs of nodes in each graph and then averaged over all graphs in the sample.
- 9.
A lattice or grid of length l and dimension d is a set of \(l^d\) points with all possible combinations of integer coordinates from 0 to \(l-1\) in all d dimensions. The most common grid is the two-dimensional grid. The points are normally connected to their \(2\cdot d\) nearest points but principally they can be connected to an arbitrary (but constant) number of nearest points.
- 10.
The diameter of a graph G is defined as the maximal distance between any two vertices in G.
- 11.
Note that the size of the grid is the number of nodes in one dimension. Since we use a two-dimensional grid, the actual number of nodes in the graph is \(size^2\). Note also that each vertex is connected to its four closest neighbors, but it could be connected to any constant number of closest neighbors and the effect would still be the same.
- 12.
Note that this is not a very strict definition. There were attempts to define small-worldness more strictly, e.g., Lehmann et al. [25]. However, the mental picture provoked by the definition of Watts and Strogatz is vivid and to my knowledge, there was never a debate whether a given network is a small-world or not.
- 13.
- 14.
I will later discuss how reliable this finding is in the first place—as protein-protein-interaction networks are especially prone to mistakes, their analysis requires extra carefulness (Sect. 10.3.1).
- 15.
See the glossary for an explanation of this mathematical jargon [p. 527, entry ‘limit of’].
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Zweig, K.A. (2016). Random Graphs and Network Models. In: Network Analysis Literacy. Lecture Notes in Social Networks. Springer, Vienna. https://doi.org/10.1007/978-3-7091-0741-6_6
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