[igraph] graph density on bipartite networks

[Simone Gabbriellini]

Hello List,

I've found a possible issue in igraph regarding density for bipartite networks. 
According to this paper:

Borgatti, S. P. and Everett, M. G. (1997). Network Analysis of 2-mode Data. 
Social Networks, 19(3):243–269.

"the standard denominators are clearly not appropriate for 2-mode data, since 
no ties are possible within vertex sets. The maximum number of ties possible 
occurs when every vertex in one set is connected to every vertex in the other. 
If the vertex sets are of size ni and no then this amounts to ni*no edges in 
the undirected case and 2*ni*no in the directed case"

What do you think about this? I guess density for bipartite graph should be 
calculated according to something like:

ecount(g) / length(V(g)[type==FALSE]) * length(V(g)[type==TRUE])

best,
Simone