# The Circuit-Cocircuit Reversal System and Torsor Structures on Spanning Trees

The Jacobian of a finite graph $G$ is a finite abelian group whose cardinality is equal to the number of spanning trees of $G$.  In this earlier post, I discussed a family of combinatorial bijections between elements of ${\rm Jac}(G)$ and the set ${\mathcal T}(G)$ of spanning trees of $G$.  I also discussed the fact that for plane graphs, these Bernardi bijections come from a natural simply transitive action of ${\rm Jac}(G)$ on ${\mathcal T}(G)$ (or, more precisely, a natural isomorphism class of such actions).

In the present post, I’d like to discuss a different family of simply transitive actions of ${\rm Jac}(G)$ on ${\mathcal T}(G)$ discovered by myself, Spencer Backman (a former student of mine), and Chi Ho Yuen (a current student of mine).  One virtue of our construction is that it generalizes in a natural way from graphs to regular matroids.  (We will mostly stick to the graphical case in this post, but will insert some comments about extensions to regular and/or oriented matroids.  A regular matroid can be thought of, rather imprecisely, as the smallest natural class of objects which contain graphs and admit a duality theory generalizing duality for planar graphs. Regular matroids are special cases of the more general concept of oriented matroids.)

One of the main new ideas in [BBY] (as we will henceforth refer to our paper) is to use the torsor ${\rm Pic}^{g-1}(G)$ as an intermediate object rather than ${\rm Pic}^{g}(G)$.  The latter is canonically isomorphic (as a ${\rm Jac}(G)$-torsor) to the set of break divisors on $G$; the former is isomorphic to the circuit-cocircuit reversal system, which we now introduce.