# sample_pa

##### Generate scale-free graphs according to the Barabasi-Albert model

The BA-model is a very simple stochastic algorithm for building a graph.

- Keywords
- graphs

##### Usage

```
sample_pa(n, power = 1, m = NULL, out.dist = NULL, out.seq = NULL,
out.pref = FALSE, zero.appeal = 1, directed = TRUE,
algorithm = c("psumtree", "psumtree-multiple", "bag"),
start.graph = NULL)
```pa(...)

##### Arguments

- n
Number of vertices.

- power
The power of the preferential attachment, the default is one, ie. linear preferential attachment.

- m
Numeric constant, the number of edges to add in each time step This argument is only used if both

`out.dist`

and`out.seq`

are omitted or NULL.- out.dist
Numeric vector, the distribution of the number of edges to add in each time step. This argument is only used if the

`out.seq`

argument is omitted or NULL.- out.seq
Numeric vector giving the number of edges to add in each time step. Its first element is ignored as no edges are added in the first time step.

- out.pref
Logical, if true the total degree is used for calculating the citation probability, otherwise the in-degree is used.

- zero.appeal
The ‘attractiveness’ of the vertices with no adjacent edges. See details below.

- directed
Whether to create a directed graph.

- algorithm
The algorithm to use for the graph generation.

`psumtree`

uses a partial prefix-sum tree to generate the graph, this algorithm can handle any`power`

and`zero.appeal`

values and never generates multiple edges.`psumtree-multiple`

also uses a partial prefix-sum tree, but the generation of multiple edges is allowed. Before the 0.6 version igraph used this algorithm if`power`

was not one, or`zero.appeal`

was not one.`bag`

is the algorithm that was previously (before version 0.6) used if`power`

was one and`zero.appeal`

was one as well. It works by putting the ids of the vertices into a bag (mutliset, really), exactly as many times as their (in-)degree, plus once more. Then the required number of cited vertices are drawn from the bag, with replacement. This method might generate multiple edges. It only works if`power`

and`zero.appeal`

are equal one.- start.graph
`NULL`

or an igraph graph. If a graph, then the supplied graph is used as a starting graph for the preferential attachment algorithm. The graph should have at least one vertex. If a graph is supplied here and the`out.seq`

argument is not`NULL`

, then it should contain the out degrees of the new vertices only, not the ones in the`start.graph`

.- ...
Passed to

`sample_pa`

.

##### Details

This is a simple stochastic algorithm to generate a graph. It is a discrete time step model and in each time step a single vertex is added.

We start with a single vertex and no edges in the first time step. Then we
add one vertex in each time step and the new vertex initiates some edges to
old vertices. The probability that an old vertex is chosen is given by
$$P[i] \sim k_i^\alpha+a$$ where \(k_i\)
is the in-degree of vertex \(i\) in the current time step (more precisely
the number of adjacent edges of \(i\) which were not initiated by \(i\)
itself) and \(\alpha\) and \(a\) are parameters given by the
`power`

and `zero.appeal`

arguments.

The number of edges initiated in a time step is given by the `m`

,
`out.dist`

and `out.seq`

arguments. If `out.seq`

is given and
not NULL then it gives the number of edges to add in a vector, the first
element is ignored, the second is the number of edges to add in the second
time step and so on. If `out.seq`

is not given or null and
`out.dist`

is given and not NULL then it is used as a discrete
distribution to generate the number of edges in each time step. Its first
element is the probability that no edges will be added, the second is the
probability that one edge is added, etc. (`out.dist`

does not need to
sum up to one, it normalized automatically.) `out.dist`

should contain
non-negative numbers and at east one element should be positive.

If both `out.seq`

and `out.dist`

are omitted or NULL then `m`

will be used, it should be a positive integer constant and `m`

edges
will be added in each time step.

`sample_pa`

generates a directed graph by default, set
`directed`

to `FALSE`

to generate an undirected graph. Note that
even if an undirected graph is generated \(k_i\) denotes the number
of adjacent edges not initiated by the vertex itself and not the total (in-
+ out-) degree of the vertex, unless the `out.pref`

argument is set to
`TRUE`

.

##### Value

A graph object.

##### References

Barabasi, A.-L. and Albert R. 1999. Emergence of scaling in
random networks *Science*, 286 509--512.

##### See Also

##### Examples

```
# NOT RUN {
g <- sample_pa(10000)
degree_distribution(g)
# }
```

*Documentation reproduced from package igraph, version 1.2.3, License: GPL (>= 2)*