Diagrammatic representation, centroid

The diagrammatic representation of the centroid density enhances one s ability to approximately evaluate the full perturbation series [3]. For example, one can focus on a class of diagrams with the same topological characteristics. The sum of such a class results in a compact analytical expression that includes infinite terms in the summation. A very useful technique in such cases is the renormalization of diagrams [57,58]. This procedure can be applied to the vertices to define the effective potential theory diagrammatically [3, 21-23] and, in doing so, an accurate approximation to the centroid density [3]. 

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