The package includes an implementation of Newman’s Reduced Mutual Information (RMI) , a version of the mutual information that is corrected for chance.

```
data(karate, package="igraphdata")
c1 <- membership(cluster_louvain(karate))
#> This graph was created by an old(er) igraph version.
#> Call upgrade_graph() on it to use with the current igraph version
#> For now we convert it on the fly...
c2 <- V(karate)$Faction
reduced_mutual_information(c1, c2, method="approximation2")
#> [1] 0.6419582
```

Just as with the standard mutual information, the RMI can be normalized as well:

The `barabasi_albert_blocks`

function produces scale-free
graphs using extended versions of the Barabási-Albert model that include
a community structure. It generates the graph by iteratively adding
vertices to an initial graph and joining them to the existing vertices
using preferential attachment (existing higher degree vertices are more
likely to receive new edges). Additionally, vertices are assigned labels
indicating community membership, and the probability of one vertex
connecting to another is affected by their community memberships
according to a fitness matrix `B`

(if a new vertex belongs to
community *i*, the probability of connecting to a vertex of
community *j* is proportional to *B_ij*).

The parameters that need to be set are `m`

the number of
new edges per step, the vector `p`

of label probabilities,
the fitness matrix `B`

(with the same dimensions as the
length of `p`

), and `t_max`

the final graph order.
The initial graph `G0`

can be set manually, but if not, an
appropriate graph will be generated with `m`

edges per
vertex, labels sampled from `p`

, and edge probabilities
proportional `B`

.

There are two variants of the model. If `type="Hajek"`

,
new edges are connected with preferential attachment to any existing
vertex but using the appropriate values of `B`

as weights
(see ). If `type="block_first"`

, new edges are connected
first to a community with probability proportional to the values of
`B`

, and then a vertex is chosen within that community with
regular preferential attachment. In this case, the resulting degree
distribution is scale-free (see ).

This is a simple example with just two communities and a graph of order 100 and size 400:

The `apply_subgraphs`

function is used internally in the
package, but has also been made available to the user because it can be
very convenient when working with clusters. It simply calls a function
`f`

on each of the communities of a graph (treated as it’s
own `igraph`

object), acting as a wrapper for the
`vapply`

function. The communities are given as a membership
vector `com`

.

For a very simple example, we call it to obtain the order of each of the factions of the karate club graph: