# R igraph manual pages

Use this if you are using igraph from R

## Scale-free random graphs, from vertex fitness scores

### Description

This function generates a non-growing random graph with expected power-law degree distributions.

### Usage

```sample_fitness_pl(
no.of.nodes,
no.of.edges,
exponent.out,
exponent.in = -1,
loops = FALSE,
multiple = FALSE,
finite.size.correction = TRUE
)
```

### Arguments

 `no.of.nodes` The number of vertices in the generated graph. `no.of.edges` The number of edges in the generated graph. `exponent.out` Numeric scalar, the power law exponent of the degree distribution. For directed graphs, this specifies the exponent of the out-degree distribution. It must be greater than or equal to 2. If you pass `Inf` here, you will get back an Erdos-Renyi random network. `exponent.in` Numeric scalar. If negative, the generated graph will be undirected. If greater than or equal to 2, this argument specifies the exponent of the in-degree distribution. If non-negative but less than 2, an error will be generated. `loops` Logical scalar, whether to allow loop edges in the generated graph. `multiple` Logical scalar, whether to allow multiple edges in the generated graph. `finite.size.correction` Logical scalar, whether to use the proposed finite size correction of Cho et al., see references below.

### Details

This game generates a directed or undirected random graph where the degrees of vertices follow power-law distributions with prescribed exponents. For directed graphs, the exponents of the in- and out-degree distributions may be specified separately.

The game simply uses `sample_fitness` with appropriately constructed fitness vectors. In particular, the fitness of vertex i is i^(-alpha), where alpha = 1/(gamma-1) and gamma is the exponent given in the arguments.

To remove correlations between in- and out-degrees in case of directed graphs, the in-fitness vector will be shuffled after it has been set up and before `sample_fitness` is called.

Note that significant finite size effects may be observed for exponents smaller than 3 in the original formulation of the game. This function provides an argument that lets you remove the finite size effects by assuming that the fitness of vertex i is (i+i0-1)^(-alpha) where i0 is a constant chosen appropriately to ensure that the maximum degree is less than the square root of the number of edges times the average degree; see the paper of Chung and Lu, and Cho et al for more details.

### Value

An igraph graph, directed or undirected.

### Author(s)

Tamas Nepusz ntamas@gmail.com

### References

Goh K-I, Kahng B, Kim D: Universal behaviour of load distribution in scale-free networks. Phys Rev Lett 87(27):278701, 2001.

Chung F and Lu L: Connected components in a random graph with given degree sequences. Annals of Combinatorics 6, 125-145, 2002.

Cho YS, Kim JS, Park J, Kahng B, Kim D: Percolation transitions in scale-free networks under the Achlioptas process. Phys Rev Lett 103:135702, 2009.

### Examples

```
g <- sample_fitness_pl(10000, 30000, 2.2, 2.3)
## Not run: plot(degree_distribution(g, cumulative=TRUE, mode="out"), log="xy")
```

[Package igraph version 1.2.7 Index]