The understanding of Cutkosky rules as analytic properties of Feynman amplitudes
involves graph insertions in many incarnations. We emphasize connections to structures
apparent in Outer Space as studied by Karen Vogtmann and collaborators.
Links:
[1] http://www.mpim-bonn.mpg.de/taxonomy/term/39
[2] http://www.mpim-bonn.mpg.de/node/3444
[3] http://www.mpim-bonn.mpg.de/node/6477