  With the network methods and random matrix theory, we investigate the
interaction structure of communities in financial markets. In particular, based
on the random matrix decomposition, we clarify that the local interactions
between the business sectors (subsectors) are mainly contained in the sector
mode. In the sector mode, the average correlation inside the sectors is
positive, while that between the sectors is negative. Further, we explore the
time evolution of the interaction structure of the business sectors, and
observe that the local interaction structure changes dramatically during a
financial bubble or crisis.
