Answer : Diagram machinery works also for perturbation theory in classical statistical mechanics and classical field theories. Generally, various kinds of diagrams constitute a pictorial way of talking about tensor products and their contractions while hiding the multi-linear algebra from the layman. In the simplest case, vertices (or blobs) represent vectors, matrices, tensors; vertices have ingoing and/or outgoing ports; ingoing ports denote (contravariant) ro indices; outgoing ports denote (covariant) co indices; directed arcs between blobs give a pair of equal indices summed over; different base spaces ⇔ \Leftrightarrow ⇔ different arc types; symmetric tensors ⇔ \Leftrightarrow ⇔ undirected diagrams; no labels are needed for internal lines. In many cases, the directed arcs are decorated (as thin, thick, broken,wavy, curly lines), each decoration indicating the presence of a so-called propagator, a function of its label to use as a weight in the sum, which may beco
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