  Expander graphs are widely used in communication problems and construction of
error correcting codes. In such graphs, information gets through very quickly.
Typically, it is not true for social or biological networks, though we may find
a partition of the vertices such that the induced subgraphs on them and the
bipartite subgraphs between any pair of them exhibit regular behavior of
information flow within or between the subsets. Implications between spectral
and regularity properties are discussed.
