Biased graphs. I. Bias, balance, and gains

A biased graph is a graph together with a class of cirles (simple closed paths), called balanced, such that no theta subgraph contains exactly two balanced circles. A gain graph is a graph in which each edge has a gain (a label from a group so that reversing the direction inverts the gain); a circle is balanced if its edge gain product is 1; this defines a biased graph. We initiate a series devoted to biased graphs and their matroids. Here we study properties of balance and also subgraphs and contractions of biased and gain graphs.