When does a biased graph come from a group labelling?

M. De Vos, D. Funk, Irene Pivotto

    Research output: Contribution to journalArticlepeer-review

    10 Citations (Scopus)


    © 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 group-labellings. 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 minor-minimal not group-labellable 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 well-quasi-ordered 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 well-quasi-ordered under minors.
    Original languageEnglish
    Pages (from-to)2510-2519
    Number of pages18
    JournalAdvances in Applied Mathematics
    Early online date14 Sept 2014
    Publication statusPublished - Oct 2014


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