Graph Bases and Diagram Commutativity
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Abstract
Given two cycles A and B in a graph, such that \(A\cap B\) is a non-trivial path, the connected sum \(A\hat{+} B\) is the cycle whose edges are the symmetric difference of E(A) and E(B). A special kind of cycle basis for a graph, a connected sum basis, is defined. Such a basis has the property that a hierarchical method, building successive cycles through connected sum, eventually reaches all the cycles of the graph. It is proved that every graph has a connected sum basis. A property is said to be cooperative if it holds for the connected sum of two cycles when it holds for the summands. Cooperative properties that hold for the cycles of a connected sum basis will hold for all cycles in the graph. As an application, commutativity of a groupoid diagram follows from commutativity of a connected sum basis for the underlying graph of the diagram. An example is given of a noncommutative diagram with a (non-connected sum) basis of cycles which do commute.
Keywords
Cycle basis Connected sum Commutative diagram Groupoid Robust cycle basis Ear basis Geodesic cycleMathematics Subject Classification
05C38 20L05 18A10Notes
Acknowledgements
We thank the referees for helpful comments.
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