Unevenness of loop location in complex networks.

The loop structure plays an important role in many aspects of complex networks and attracts much attention. Among the previous works, Bianconi [Phys. Rev. Lett. 100, 118701 (2008)] found that real networks often have very few short loops as compared to random models. In this paper, we focus on the uneven location of loops which makes some parts of the network rich while some other parts sparse in loops. We propose a node removing process to analyze the unevenness and find rich loop cores can exist in many real networks such as neural networks and food web networks. Finally, an index is presented to quantify the unevenness of loop location in complex networks.