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Abstract
© 2014 Elsevier Inc. A biased graph consists of a graph G together with a collection of distinguished cycles of G, called balanced, with the property that no theta subgraph contains exactly two balanced cycles. Perhaps the most natural biased graphs on G arise from orienting G and then labelling the edges of G with elements of a group Γ. In this case, we may define a biased graph by declaring a cycle to be balanced if the product of the labels on its edges is the identity, with the convention that we take the inverse value for an edge traversed backwards. Our first result gives a natural topological characterisation of biased graphs arising from grouplabellings. In the second part of this article, we use this theorem to construct some exceptional biased graphs. Notably, we prove that for every m≥3 and there exists a minorminimal not grouplabellable biased graph on m vertices where every pair of vertices is joined by at least edges. In particular, this shows that biased graphs are not wellquasiordered under minors. Finally, we show that these results extend to give infinite sets of excluded minors for certain natural families of frame and lift matroids, and to show that neither are these families wellquasiordered under minors.
Original language  English 

Pages (fromto)  25102519 
Number of pages  18 
Journal  Advances in Applied Mathematics 
Volume  61 
Early online date  14 Sep 2014 
DOIs  
Publication status  Published  Oct 2014 
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Projects
 1 Finished

Exact Structure in Graphs & Matroids
Royle, G., Mayhew, D. & Whittle, G.
1/01/11 → 31/12/13
Project: Research