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Cartoon of a 3-core network k-shells are defined as the set of nodes that belong to the k-core but not the k + 1-core. As illustrated, k-shells are concentric; low k-shells are located in the outer part of the network, while for increasing k, nodes are situated in the most focal part until one reaches the highest k-shell, which corresponds to the k-core, and is located at the center of the graph
