We show that any cocycle from an ergodic, finite measure preserving action of a higher-rank group to a closed subgroup of the isometry group of a proper, geodesic hyperbolic, ‘at most exponential’ metric space is necessarily cohomologous to a cocycle with values in a compact subgroup. Philosophically, this says that higher-rank dynamics is incompatible with hyperbolic dynamics; since hyperbolicity is, in some sense, generic among finitely presented groups, this places strong restrictions on the possible dynamics of higher-rank groups. We derive some applications of the main result. When the target group of the cocycle has no small subgroups, we show that the main result holds for a wider class of domain groups.

We study a generic, real analytic unfolding of a planar diffeomorphism having a fixed point with unipotent linear part. In the analogue for vector fields an open parameter domain is known to exist, with a unique limit cycle. This domain is bounded by curves corresponding to a Hopf bifurcation and to a homoclinic connection. In the present case of analytic diffeomorphisms, a similar domain is shown to exist, with a normally hyperbolic invariant circle. It follows that all the ‘interesting’ dynamics, concerning the destruction of the invariant circle and the transition to trivial dynamics by the creation and death of homoclinic points, takes place in an exponentially small part of the parameter-plane. Partial results were stated in [5]. Related numerical results appeared in [16].

We prove that for almost every (with respect to the Lebesgue measure) a ∈ [√2, 2], the forward trajectory of the turning point of the tent map fa with slope a is dense in the interval of transitivity of fa.

We give examples of non-integrable measurable functions for which there are ‘many’ rotations such that the arithmetic (ergodic) averages exist for almost every x. We also show that if the above ergodic averages exist for almost every x for a set of rotations of positive measure, then the function should be integrable on [0, 1].

We give a condition for automorphisms α and β on finite von Neumann algebras which induces the tensor product formula for entropies: H(α ⊗ β) = H(α) + H(β). As an application, the Bernoulli shift (1/n, 1/n, …, 1/n) has extensions to ergodic outer automorphisms {αk; k = 1,2, …} on the hyperfinite II1 factor R with the entropies H(αk) = (1/2)kn log n.

In this paper, structural stability of discrete-time linear random dynamical systems is studied. A random dynamical system is called structurally stable with respect to a random norm if it is topologically conjugate to any random dynamical system which is sufficiently close to it in this norm. We prove that a discrete-time linear random dynamical system is structurally stable with respect to its Lyapunov norms if and only if it is hyperbolic.

Let G be a countable group of automorphisms of a Lebesgue space (X, m) and let [G] be the full group of G. For a pair of countable ergodic subgroups H1 and H2 of [G], the following problem is considered: when are the full subgroups [H1] and [H2] conjugate in the normalizer N[G] = {g ∈ Aut X: g[G]g-1 = [G]} of [G]. A complete solution of the problem is given in the case when [G] is an approximately finite group of type II and [H] is admissible, in the sense that there exists an ergodic subgroup [H0] of [G] and a countable subgroup Γ ⊂ N[H0] consisting of automorphisms which are outer for [H0], such that [H0] ⊂ [G] and the full subgroup [Ho, Γ] generated by [H0] and Γ coincides with [G].

We develop a new type of backward continued fractions that can be associated to each Hecke-type group. We study its symbolic dynamics, and the corresponding interval maps and their invariant measures. These measures are infinite if and only if the corresponding groups are discrete. For the discrete Hecke groups the invariant measure is computed explicitly by studying the geodesic flow on the associated Riemann surface.

We give a definition for a Julia set J(f) for generic classes of polynomial endomorphisms f: ℂn→ ℂn. For n = 1, our definition is equivalent to the usual one, which gives the points where the iterates of f do not form a normal family. Moreover, the Julia set J(f1 × … × fn) ⊂ ℂn for a product of one-dimensional polynomials fi: ℂ → ℂ turns out to be the product J(f1) × … × J(fn) of the associated Julia sets J(fi) ⊂ ℂ. For a special class of mappings f: ℂ2 → ℂ2 which is not of this simple type, the so-called Cantor skews, we investigate topological structure as well as measure theoretic aspects of the Julia sets obtained using our definition.

We describe several generic properties of homeomorphisms on compact manifolds. These properties concern the attractors of a homeomorphism, its chain recurrent set, and the periodic points.

We prove for a large class of compact metric spaces, including those manifolds of dimension at least two, Hilbert cube manifolds, and homogeneous Menger manifolds, that ‘most’ self-homeomorphisms (in the sense of residual set of homeomorphisms) have certain properties. Specifically, if F: X → X is one of these homeomorphisms, then F admits

• a dense, open wandering set;

• a nowhere dense chain recurrent set;

• an infinite collection of attractors (and repellers), each of which has nonempty interior and cannot be reduced to a ‘smallest’ attractor (or ‘largest’ repeller); and an uncountable collection of pairwise disjoint quasi-attractors.

We also discuss the topology of the boundaries of attractors.

We prove: If f(z) is a critically finite rational map which has exactly two critical points and which is not conjugate to a polynomial, then the boundary of every Fatou component of f is a Jordan curve. If f(z) is a hyperbolic critically finite rational map all of whose postcritical points are periodic, then there exists a cycle of Fatou components whose boundaries are Jordan curves. We give examples of critically finite hyperbolic rational maps f with a Fatou component ω satisfying f(ω) = ω and f|∂ω not topologically conjugate to the dynamics of any polynomial on its Julia set.

The following theorem was published in [2].

Theorem. There exists a constant δ > 0 such that for Lebesgue almost every (α, β) ∈ X = [0, 1] × [0, 1], there exists no = no(α, β) such that for any n > no

where the integers pn, qn, rn are provided by the modified Jacobi-Perron algorithm.