Many of the phenomena in the complex world in which we live have a rough description as a large network of interacting components. Random network theory tries to describe the global structure of such networks from basic local principles. One such principle is the preferential attachment paradigm which suggests that networks are built by adding nodes and links successively, in such a way that new nodes prefer to be connected to existing nodes if they have a high degree. Our research gives the first comprehensive and mathematically rigorous treatment of the case when this preference follows a nonlinear, or more precisely concave, rule. We survey results obtained so far and some ongoing developments.
NetworksAlthough networks are in principle simple objects, they can show immense complexity when their size is very large. They can therefore often offer a meaningful rough description of complex systems in nature, society or technology. Examples of the kind of networks we have in mind are the following:• a cell may be described by its metabolism as a network of chemicals with edges representing chemical reactions transforming one substance into another one;• a social network has human individuals as nodes and edges representing either friendship, acts of communication or other social interactions;• in a collaboration graph the nodes represent scientists which are connected by an edge if they have collaborated or if they have authored a joint paper;• the world-wide web consists of web pages connected by hyperlinks;• the internet has routers and computers as nodes which have physical or wireless links.The mathematical notion of networks is easy to derive.
Definition 1.A network is defined as a finite set V of nodes, or vertices, together with a set E ⊂ V × V of links, or edges. The degree of a vertex v ∈ V is the number of vertices w ∈ V with (w, v) ∈ E (called the outdegree) plus the number of vertices w ∈ V with (v, w) ∈ E (called the indegree). Two vertices v, w ∈ V are connected if there exist finitely many vertices v = v 0 , v 1 , . . . , v n = w such that for every i ∈ {1, . . . , n} we have (v i−1 , v i ) ∈ E or (v i , v i−1 ) ∈ E. This defines an equivalence relation on V and therefore a partition of the set V of vertices into connected components.