John Baez's Prehistory of n-Categorial Physics [1] has some interesting nuggets about Feynman diagrams, in the "Feynman (1947)" section. In particular, "The ma

John Baez's Prehistory of n-Categorial Physics [1] has some interesting nuggets about Feynman diagrams, in the "Feynman (1947)" section. In particular, "The mathematics necessary for [interesting Feynman diagrams] was formalized later, in Mac Lane's 1963 paper on monoidal categories (see below) and Joyal and Street's 1980s work on 'string diagrams.'" In other words, Feynman diagrams were (or at least can be taken as) an early precursor of diagrammatic approaches in category-theoretic mathematical physics. The paper is really great for getting some context around these diagrams and a sense of the underlying mathematics, even for a category-theory ignoramus like myself, with a little "suspension of comprehension." 1: http://math.ucr.edu/home/baez/history.pdf http://math.ucr.edu/home/baez/history.pdf