Published online by Cambridge University Press: 02 October 2001
The unstable manifolds of hyperbolic systems admitting a one-rectangle Markov partition are here characterized up to homeomorphism and up to conjugacy of the underlying dynamics in a very easy-to-compute way. We associate to each one-rectangle system a word over the alphabet \{+,-\} describing the bending of the image rectangle with respect to the initial rectangle. We endow the set \{+,-\}^{\ast} of such words with a product \wedge having a dynamical origin. The structure of the non-commutative semigroup (\{+,-\}^{\ast},\wedge) is made completely explicit. The topological and dynamical properties of the unstable manifolds of one-rectangle systems are translated in terms of the decomposition of the associated words in such a semigroup.