In each of the ten cases with zero or unit mass, the finite part of the scalar 3-loop tetrahedral vacuum Feynman diagram reduces to four-letter "words" that
represent iterated integrals in an alphabet of seven "letters" (Broadhurst
1998, Bailey et al. 2007). There are such four-letter words, but only two of these are primitive
terms. Defining
Bailey, D. H.; Borwein, J. M.; Calkin, N. J.; Girgensohn, R.; Luke, D. R.; and Moll, V. H. "Quantum Field Theory."
§2.3.5 in Experimental
Mathematics in Action. Wellesley, MA: A K Peters, pp. 40-41, 2007.Broadhurst,
D. J. "Massive 3-Loop Feynman Diagrams Reducible to Primitives of Algebras of the Sixth Root of Unity."
March 11, 1998. http://arxiv.org/abs/hep-th/9803091.