Biased graphs. VII. Contrabalance and antivoltages

We develop linear representation theory for bicircular matroids, a chief example being a matroid associated with forests of a graph, and bicircular lift matroids, a chief example being a matroid associated with spanning forests. (These are bias and lift matroids of contrabalanced biased graphs.) The theory is expressed largely in terms of antivoltages (edge labellings that defy Kirchhoff’s voltage law) with values in the multiplicative or additive group of the scalar field. We emphasize antivoltages with values in cyclic groups and finite vector spaces since they are crucial for representing the matroids over finite fields; and integer-valued antivoltages with bounded breadth since they are crucial in constructions. We find bounds for the existence of antivoltages and we solve some examples. Other results: The number of antivoltages in an abelian group is a polynomial function of the group order, and the number of integral antivoltages with bounded breadth is a polynomial in the breadth bound. We conclude with an application to complex representation. There are many open questions.