Boolean network

Classical model

A Boolean network is a particular kind of sequential dynamical system, where time and states are discrete, i.e. both the set of variables and the set of states in the time series each have a bijection onto an integer series. Boolean networks are related to cellular automata.

A random Boolean network (RBN) is one that is randomly selected from the set of all possible boolean networks of a particular size, N. One then can study statistically, how the expected properties of such networks depend on various statistical properties of the ensemble of all possible networks. For example, one may study how the RBN behavior changes as the average connectivity is changed.

The first Boolean networks were proposed by Stuart A. Kauffman in 1969, as random models of genetic regulatory networks but their mathematical understanding only started in the 2000's.

Attractors

Since a Boolean network has only 2^N possible states, a trajectory will sooner or later reach a previously visited state, and thus, since the dynamics are deterministic, the trajectory will fall into a cycle. In the literature in this field, each cycle is also called an attractor (though in the broader field of dynamical systems a cycle is only an attractor if perturbations from it lead back to it). If the attractor has only a single state it is called a point attractor, and if the attractor consists of more than one state it is called a cycle attractor. The set of states that lead to an attractor is called the basin of the attractor. States which occur only at the beginning of trajectories (no trajectories lead to them), are called garden-of-Eden states and the dynamics of the network flow from these states towards attractors. The time it takes to reach an attractor is called transient time.

With growing computer power and increasing understanding of the seemingly simple model, different authors gave different estimates for the mean number and length of the attractors, here a brief summary of key publications.

Stability

The stability of Boolean networks depends on the connections of their nodes. A Boolean network can exhibit stable, critical or chaotic behavior. This phenomenon is governed by a critical value of the average number of connections of nodes ( K c ), and can be characterized by the Hamming distance as distance measure. In the unstable regime, the distance between two initially close states on average grows exponentially in time, while in the stable regime it decreases exponentially. In this, with "initially close states" one means that the Hamming distance is small compared with the number of nodes ( N ) in the network.

For N-K-model the network is stable if K < K c , critical if K = K c , and unstable if K > K c .

The state of a given node n i is updated according to its truth table, whose outputs are randomly populated. p i denotes the probability of assigning an off output to a given series of input signals.

If p i = p = c o n s t . for every node, the transition between the stable and chaotic range depends on p . The critical value of the average number of connections is K c = 1 / [ 2 p ( 1 − p ) ] .

If K is not constant, and there is no correlation between the in-degrees and out-degrees, the conditions of stability is determined by ⟨ K i n ⟩ The network is stable if ⟨ K i n ⟩ < K c , critical if ⟨ K i n ⟩ = K c , and unstable if ⟨ K i n ⟩ > K c .

The conditions of stability are the same in the case of networks with scale-free topology where the in-and out-degree distribution is a power-law distribution: P ( K ) ∝ K − γ , and ⟨ K i n ⟩ = ⟨ K o u t ⟩ , since every out-link from a node is an in-link to another.

Sensitivity shows the probability that the output of the Boolean function of a given node changes if its input changes. For random Boolean networks, q i = 2 p i ( 1 − p i ) . In the general case, stability of the network is governed by the largest eigenvalue λ Q of matrix Q , where Q i j = q i A i j , and A is the adjacency matrix of the network. The network is stable if λ Q < 1 , critical if λ Q = 1 , unstable if λ Q > 1 .

Other topologies

One theme is to study different underlying graph topologies.

Other updating schemes

Classical Boolean networks (sometimes called CRBN, i.e. Classic Random Boolean Network) are synchronously updated. Motivated by the fact that genes usually not simultaneously changing their state, different alternatives have been introduced. A common classification is the following: