We call a Markov partition of a two dimensional hyperbolic toral automorphism a Berg partition if it contains just two rectangles. We describe all Berg partitions for a given hyperbolic toral automorphism. In particular there are exactly (k + n + l + m)/2 nonequivalent Berg partitions with the same connectivity matrix (k, l, m, n).
arXiv:1007.3984v1 [math.DS] 22 Jul 2010At least one of the rectangles in a bi-partion is connected.Definition 2.2 For a given bi-partition the union of the horizontal sides, J s ⊂ T 2 , of the rectangles is called the horizontal spine, the union of the vertical sides, J u ⊂ T 2 , is called the vertical spine.The horizontal and the vertical spines are intervals in T 2 which intersect in four points J s ∩ J u , Fig.1.Let us now reverse the question, given two transversal irrational directions in the torus, let us find all bi-partitions with the sides of the rectangles having these directions. In other words, let L ⊂ R 2 be a lattice of translations isomorphic to Z 2 , and with neither horizontal nor vertical translations, i.e., the lattice L has no nonzero elements in the coordinate axes. We are looking for two rectangles R 1 , R 2 , which form the bi-partition of the torus R 2 /L. It is clear from the above construction that such bi-partitions are, one-to-one, associated with bases {e, f } of the lattice L, such that e belongs to the first, and f belongs to the second quadrant. Indeed, if e = [v, p] and f = [−u, q], then the rectangles R 1 , R 2 , with the horizontal sides equal to, respectively, u and v, and vertical sides equal to p and q, Fig. 1, give us the bi-partition.Let us consider the family F of such bases of L. F is always nonempty. Indeed, let {a, b} be a basis in L. One of the four bases {±a, ±b} has the property that the first element, with respect to the positive orientation, is in