Bianconi–Barabási model

Concepts

The Barabási–Albert (BA) model uses two concepts: growth and preferential attachment. Here, growth indicates the increase in the number of nodes in the network with time, and preferential attachment means that more connected nodes receive more links. The Bianconi–Barabási model, on top of these two concepts, uses another new concept called the fitness. This model makes use of an analogy with evolutionary models. It assigns an intrinsic fitness value to each node, which embodies all the properties other than the degree. The higher the fitness, the higher the probability of attracting new edges. Fitness can be defined as the ability to attract new links – "a quantitative measure of a node's ability to stay in front of the competition".

While the Barabási–Albert (BA) model explains the "first mover advantage" phenomenon, the Bianconi–Barabási model explains how latecomers also can win. In a network where fitness is an attribute, a node with higher fitness will acquire links at a higher rate than less fit nodes. This model explains that age is not the best predictor of a node's success, rather latecomers also have the chance to attract links to become a hub.

The Bianconi–Barabási model can reproduce the degree correlations of the Internet Autonomous Systems. This model can also show condensation phase transitions in the evolution of complex network.

While the BA model is useful in growing networks, the Bianconi–Barabási model also works in case of static networks where the number of nodes is fixed. The BB model can predict the topological properties of Internet.

Algorithm

The fitness network begins with a fixed number of interconnected nodes. They have different fitness, which can be described with fitness parameter, η j which is chosen from a fitness distribution ρ(η).

Growth

The assumption here is that a node’s fitness is independent of time, and is fixed. A new node j with m links and a fitness η j is added with each time-step.

Preferential attachment

The probability Πi that a new node connects to one of the existing links to a node i in the network depends on the number of edges, k i , and on the fitness η i of node i, such that,

Each node’s evolution with time can be predicted using the continuum theory. If initial number of node is m, then the degree of node i changes at the rate:

Assuming the evolution of k i follows a power law with a fitness exponent β ( η i )

Here, β ( η ) = η C  and  C = ∫ ρ ( η ) η 1 − β ( η ) d η .

Equal fitnesses

If all fitnesses are equal in a fitness network, the Bianconi–Barabási model reduces to the Barabási–Albert model.

When fitnesses are equal, the probability P i i that the new node is connected to node i when k i is the degree of node i is,

Degree distribution

Degree distribution of the Bianconi–Barabási model depends on the fitness distribution ρ(η). There are two scenarios that can happen based on the probability distribution. If the fitness distribution has a finite domain, then the degree distribution will have a power-law just like the BA model. In the second case, if the fitness distribution has an infinite domain, then the node with the highest fitness value will attract a large number of nodes and show a winners-take-all scenario. As the Bianconi–Barabási model does not always show the power-law distribution, it has certain advantages such that this model can be used in non-heterogeneous networks such as World Trade Web.

History

In 1999, Albert-László Barabási requested his student Bianconi to investigate evolving networks where nodes have a fitness parameter. Barabási was interested in finding out how Google, a latecomer in the search engine market, became a top player. Google’s toppling of previous top search engines went against Barabási's BA model, which states that first mover has an advantage. In the scale-free network if a node appears first it will be most connected because it had the longest time to attract links. Bianconi's work showed that when fitness parameter is present, the "early bird" is not always the winner. Bianconi and Barabási's research showed that fitness is what creates or breaks the hub. Google's superior PageRank algorithm helped them to beat other top players. Later on Facebook came and dethroned Google as Internet's most linked website. In all these cases fitness mattered which was first showed in Bianconi and Barabási's research. In 2001, Ginestra Bianconi and Albert-László Barabási published the model in the Europhysics Letters. In another paper, substituting fitness for energy, nodes for energy level and links for particles, Bianconi and Barabási was able to map the fitness model with Bose gas.