The Active Bijection 2.b - Decomposition of activities for oriented matroids, and general definitions of the active bijection.

The active bijection for oriented matroids (and real hyperplane arrangements, and graphs, as particular cases) is introduced and investigated by the authors in a series of papers. Given any oriented matroid defined on a linearly ordered ground set, we exhibit one particularitu0027e of its bases, which we call its active basis, with remarkable properties. It preserves activities (for oriented matroids in the sense of Las Vergnas, for matroid bases in the sense of Tutte), as well as some active partitions of the ground set associated with oriented matroids and matroid bases. It yields a canonical bijection between classes of reorientations and bases [...]. It also yields a refined bijection between all reorientations and subsets of the ground set. Those bijections are related to various Tutte polynomial expressions [...]. They contain various noticeable bijections involving orientations/signatures/reorientations and spanning trees/simplices/bases of a graph/real hyperplane arrangement/oriented matroid. [...] In previous papers of this series, we defined the active bijection between bounded regions and uniactive internal bases by means of fully optimal bases (No. 1), and we defined a decomposition of activities for matroid bases by means of [...] particular sequences of minors (companion paper, No. 2.a). The present paper is central in the series. First, we define a decomposition of activities for oriented matroids, using the same sequences of minors, yielding a decomposition of an oriented matroid into bounded regions of minors. Second, we use the previous results together to provide the canonical and refined active bijections alluded to above. We also give an overview and examples of the various results of independent interest involved in the construction. They arise as soon as the ground set of an oriented matroid is linearly ordered.