 - 9 mins

# Networks

Complex system : composed of interconnected parts that as a whole exhibit one or more properties not obvious from the properties of the individul parts

## Graphs and networks

Graph : vertices, edges, directed or not, weighted or not

Network : nodes, links, flows, epidemics, growth, rewiring, etc.

The next step from graphs to networks is complex systems which we model with networks!

#### Graph types

• simple graph: only single connections, no self loops
• multi graph: multiple connections, no self loops
• pseudo graph: self loops allowed
• bi-partite graph: two types of nodes, different are linked

Sotring it on computer can be done in an adjecency matrix (not memory efficient) or in an adjecency list (linked list).

$A_{ij} = \Big( 1 ~ if ~ i ~ \rightarrow j ~, ~ 0 ~ otherwise \Big)$

For undirected graphs A is symmetric. Multiplying a position vector with A gives the neighbors of that point.

#### Sparseness

A network is sparse in the $$N \rightarrow \infty$$ limit if the number of links $$M \propto N$$.

A networks is dense in the $$N \rightarrow \infty$$ limit if the number of links $$M \propto N^{2}$$.

In the dense case the average connections per node goes to infinity. ($$\langle k \rangle = \frac{2M}{N}$$).

Most real networks are sparse. Therefore the probability of two nodes being linked is negligible, goes to 0 in the $$N \rightarrow \infty$$ limit.

### Basics of networks

• number of connections per node: $$k_{i}$$, if directed $$k_{i}^{in}$$, $$k_{i}^{out}$$
• weighted networks have node strenght: $$s_{i}$$, a node’s strenght is the biggest weight of the edges connecting it to other nodes
• avarage degree: $$\langle k \rangle = \frac{1}{N}\sum_{i = 1}^{N}k_{i} = \frac{2M}{N}$$, for directed networks $$\langle k_{in} \rangle = \langle k_{out} \rangle = \frac{M}{N}$$
• clustering coefficient: $$C_{i} = \frac{2e_{i}}{k_{i}(k_{i} - 1)}$$ where $$e_{i}$$ is the number of links between its neighbors, in the directed case $$C_{i}/2 \rightarrow C_{i}$$, one can also the the average clustering coeff. of a networks as well

• $$\langle k \rangle$$ is the measure of global densitiy of the network, therefore $$\langle C \rangle$$ is a local density measure

#### Pathology

• Cycle: path with same start and end node
• Eulerian path: a path traversing each link exactly onces
• Hamiltonian path: a path visiting eaxh node exactly once
• distance : $$l_{ij}$$ minimum number of steps it takes to reach j from i, in undirected networks this matrix is symmetric and if j cannot be reached from i then $$l_{ij} = \infty$$

• avarage shortest path lenght: $$\langle l \rangle = \frac{2}{N(N-1)}\sum_{i \langle j}l_{ij}$$ and for a given node $$\langle l_{i} \rangle = \frac{1}{N - 1}\sum_{j}l_{ij}$$

• for sparse networks where N is large and $$\langle k \rangle$$ is small a good estimate is $$\langle l \rangle = \frac{ln N}{ln \langle k \rangle }$$
• small world property : $$\langle l \rangle \propto lnN$$ this always results in a small average shortest path lenght for big system sizes
• diameter: $$D = max_{ij} \{ l_{ij} \}$$
• centrality : $$C_{c}(i) = \frac{1}{\langle l_{i}\rangle }$$ where unreachable nodes are not considered in the sum $$\rightarrow$$ intuitively a node closer to the rest of the network is more central

• betweenness : of a node or link is equal to the number of shortest path passing through a given node or link, if multiple shortest paths are possible between a given pair of nodes then they are given equal weidghts and adding up to one:
$b_{i} = \sum_{v \neq i, s \neq i} \frac{\sigma_{sv}(i)}{\sigma_{sv}}$

#### Algorithms

• depth first search
• Dijkstra’s algorithm

#### PageRank

The importance of a node is affected by the number of in-neighbors and the importance of the in-neighbors.

Assume an iteraive process, each node distributes its PageRank evenly at each timestep between its neighbors.

$r_{i}(t + 1) = \sum_{j ~ \in ~ Negihbors(i)} \frac{r_{j}(t)}{k_{j}}$

Total PageRank of the system is therefore conserved since it is only being redistributed at each timestep. For simplicity one should scale it to 1.

If we intruduce dampening with probability $$p_{d}$$:

$r_{i}(t + 1) = \frac{p_{d}}{N} + (1 - p_{d})\sum_{j ~ \in ~ Negihbors(i)} \frac{r_{j}(t)}{k_{j}}$

#### Components

A component is an undirected network that corresponds to a maximal set of nodes in which a path exists between any pair of nodes.

Most networks contain a giant component.

A giant component in a network is a subsystem which relative size remains finite in the $$N \rightarrow \infty$$ limit.

$lim_{N \rightarrow \infty} \frac{S_{1}}{N} \rangle 0$

Can be generalized to the directed case:

• strongly connected component: is a maximal set of nodes in which a directed path exists between any pair of nodes
• weakly connected component: is a maximal set of nodes in which an undirected path exist between any pair of nodes

• degree distribution $$p(k)$$
• for a finite netork with N nodes: $$p(k) = \frac{N_{k}}{N}$$ where $$N_{k}$$ is the number of nodes with degree k
• for an Erdős-Rényi graph where links are established with probability $$p$$ and not established with probablity $$1-p$$ for small network size N the degree dstribution is approximately binomial but in the $$N \rightarrow \infty$$ limit it is a Poisson-distribution with $$\langle k\rangle = Np$$
$p(j) \approx \frac{\langle k\rangle ^{j}}{j!}e^{-\langle k\rangle }$

### Scale-free networks

A network is scale-free if the tail of its degree distribution decays as a power-law, where $$\gamma$$ is the node degree decay exponent:

$p(k) \propto k^{-\gamma}$

It is called scale free because scaling functions are defined as:

$F(a\cdot x) = g(a)\cdot F(x)$

and power laws are scaling.

Wealth densitiy is a scaling probability density ( $$\rho(x) \propto x^{-\alpha}$$ ), this is called the Pareto-distribution.

Also there is no typical degree in scale-free networks as a result of being scaling.

Normalizing the scale-free degree distribution:

$\int_{k_{min}}^{\infty} p(k)dk = \int_{k_{min}}^{\infty} Ck^{-\gamma}dk = 1$

Therefore $$C$$ is:

$C = \frac{\gamma - 1}{k_{min}^{1 - \gamma}}$

As a result:

$p(k) = (\gamma - 1)\frac{k^{-\gamma}}{k_{min}^{1 - \gamma}}$

Scale-freeness implies large hubs in the network.

In physics being scale-free usually occurs at phase transitions at critical points and often suggests divergence. The moments of p(k) are divergent for a scale-free system when $$m \rangle \gamma - 1$$.

Most real world networks are scale-free and their node degree decay factor ($$\gamma$$) is below 3 thus $$\langle k^{2}\rangle$$ is divergent in the $$N \rightarrow \infty$$ limit, therefore the standard deviation is divergent as well!

If $$\gamma$$ is in the range $$]2; 3[$$ the system behaves ultra-small world and the scale-free property is relevant.

• assortative network: small degree nodes tend to link to small degree nodes and hubs tend to link to each other
• diasortative network: hubs do not link, they connect to small degree nodes
• neutral network: random

Let $$P(k' | k)$$ denote the conditional probability of finding a node with degree $$k'$$ from a node with degree $$k$$. From the definition of conditional probability:

$P(k' | k ) = \frac{P(k, k')}{P(k)}$

Where it is easier to count all the nodes denoted with $$E_{k, k'}$$ and then dividing by the nodes that all have $$k$$ degrees $$\sum_{k'}E_{k, k'}$$.

$P(k' | k) = \frac{E_{k, k'}}{\sum_{k'}E_{k, k'}}$

If we divide $$E_{k, k'}$$ (the matrix of number of links between degree $$k$$, $$k'$$ node pairs) with the total number of links in the networks beging $$2M$$ we can get $$P(k, k')$$ which obviously sums to one for both degrees:

$\sum_{k, k'} P(k, k') = 1$ $\sum_{k'} P(k, k') = q_{k} \quad \sum_{k} P(k, k') = q_{k'}$

The meaning of $$q_{k}$$ is the probability of finding a node on the end of a link with degree $$k$$. This is obviously proportional to the number of links from nodes with degree $$k$$:

$q_{k} \propto kN_{k} \quad \rightarrow \quad q_{k} = \frac{kN_{k}}{\sum_{k'}k'N_{k'}} = \frac{kp(k)N}{\sum_{k'}k'p(k')N} = \frac{kp(k)}{\langle k\rangle }$

For a neutral network we expect $$P(k, k') = P(k) P(k')$$ which after the normalization of $$q_{k}$$ and $$q_{k'}$$ translates to $$P(k, k') = q_{k}q_{k'}$$ therefore any significant deviance from this value indicates degree correlations in a networks.

In practice preparing $$P(k, k')$$ is difficult and computationally expensive so average nearest neighbor degree is calculated instead:

$k_{i}^{ANND} = \langle k_{j}\rangle _{j ~ linked ~ to ~ i} = \frac{1}{k_{i}}\sum_{j ~ linked ~ to ~ i} k_{j}$

In terms of $$P(k' \mid k)$$, $$k^{ANND}(k) = \sum_{k'}P(k' \mid k)k' = \frac{\sum_{k'}k'P(k', k)}{q_{k}}$$.

In case of a neutral network $$P(k, k') = P(k)P(k')$$ and therefore:

$k^{ANND}(k) = \frac{\sum_{k'}k'P(k', k)}{q_{k}} = \frac{\sum_{k'}k'P(k')P(k)}{P(k)}$

We should remeber that $$P(k')$$ is the normalized $$q_{k'}$$ which is $$\frac{k'p(k')}{\langle k\rangle }$$ therefore:

$k^{ANND}(k) = \frac{\langle k^{2}\rangle }{\langle k\rangle }$

Calculating this parameter is takes $$k$$ variables in memory and less steps than calculating $$P(k, k')$$. We can now calculate the covariance of $$k, k'$$:

$Cov_{~ P(k, k') ~ }(k, k') ~ = ~ \langle kk'\rangle _{~ P(k, k') ~ } - \langle k'\rangle _{~ P(k, k') ~ }\langle k\rangle _{~ P(k, k') ~ }$

Substituting the joint probability in the above equation:

$Cov_{~ P(k, k') ~ }(k, k') ~ = ~ \sum_{k, k'}k'kP(k, k') - \Big(\sum_{k, k'}kP(k, k')\Big)\Big(\sum_{k, k'}k'P(k, k')\Big)$

After simplification:

$Cov_{~ P(k, k') ~ }(k, k') ~ = ~ \sum_{k, k'}kk'\Big(P(k, k') - P(k)P(k')\Big)$

We can simply check this formula by remembering that in a neutral network the probabilities in the parantheses cancel out and there is no co-variance between degrees.

The Pearson-correlation is defined as the co-variance of the parameters diveded by their variances. The variance of $$k$$ and $$k'$$ is:

$\sigma^{2}_{~ P(k, k') ~}(k) = \langle k^{2}\rangle _{~ P(k, k') ~} - \langle k\rangle ^{2}_{~ P(k, k') ~} = \sum_{k} k^{2}q_{k} - \Big(\sum_{k}q_{k}\Big)^{2}$

The Pearson-correlation is therefore:

$r = \frac{Cov_{~ P(k, k') ~}(k, k')}{\sigma_{~ P(k, k') ~}(k')\sigma_{~ P(k, k') ~}(k)} = \frac{\sum_{k, k'}kk'\Big(P(k, k') - P(k)P(k')\Big)}{\sigma_{~ P(k, k') ~}^{2}(k)}$

If $$r = 0$$ the network is neutral, if $$r \langle 0$$ it is disassortative otherwise assortative.