Illustration of k-core and k-core layer. The 1-core layer are the red nodes, the 2-core layer are green. The 1-core is the whole graph, the 2-core is subgraph consisting of green and blue nodes and the 3-core is the subgraph consisting of blue nodes.
Zhao et al. BMC Systems Biology 2012 6(Suppl 1):S4 doi:10.1186/1752-0509-6-S1-S4