Some Remarks on Non-Planar Diagrams

Understand

Two criteria for planarity of a Feynman diagram upon its propagators (momentum flows) are presented.

• Instructive Mathematica programs that solve the problem and examples are provided.
• A simple geometric argument is used to show that while one can planarize non-planar graphs by embedding them on higher-genus surfaces (in the example it is a torus), there is still a problem with defining appropriate dual variables since the corresponding faces of the graph are absorbed by torus generators.