We study the center problem for the class ${\mathcal{E}}_{{\rm\Gamma}}$ of Abel differential equations $dv/dt=a_{1}v^{2}+a_{2}v^{3}$, $a_{1},a_{2}\in L^{\infty }([0,T])$, such that images of Lipschitz paths $\tilde{A}:=(\int _{0}^{\cdot }a_{1}(s)\,ds,\int _{0}^{\cdot }a_{2}(s)\,ds):[0,T]\rightarrow \mathbb{R}^{2}$ belong to a fixed compact rectifiable curve ${\rm\Gamma}$. Such a curve is said to be universal if whenever an equation in ${\mathcal{E}}_{{\rm\Gamma}}$ has center on $[0,T]$, this center must be universal, i.e. all iterated integrals in coefficients $a_{1},a_{2}$ of this equation must vanish. We investigate some basic properties of universal curves. Our main results include an algebraic description of a universal curve in terms of a certain homomorphism of its fundamental group into the group of locally convergent invertible power series with the product being the composition of series, explicit examples of universal curves, and approximation of Lipschitz triangulable curves by universal ones.

1. Let ${\bf r}=(r_1,r_2,\dots)$ be a sequence of integers greater than one. Then every ${\bf r}$-adic reverse filtration has a maximal standard orbit factor.

2. For ‘entropy-free’ ${\bf r}$, $a\geqq 0$, and any free action $A$ of the group $G=(Z/r_1Z)+(Z/r_2Z)+\dotsb$, there is a free ergodic action $A'$ of $G$ with $h(A')=a$ and such that the reverse filtration produced by $A'$ is isomorphic to that produced by $A$.

We consider in this paper the space of all orientation preserving and fixed point free homeomorphisms of the plane, endowed with the compact-open topology. Our main theorem asserts that this space is locally contractible. Then we give a new proof of the path connectedness. Finally, it is shown that the inclusion of this space into the space of all orientation preserving planar homeomorphisms induces the trivial map on the fundamental groups.

We show that for every non-elementary hyperbolic group the Bowen–Margulis current associated with a strongly hyperbolic metric forms a unique group-invariant Radon measure class of maximal Hausdorff dimension on the boundary square. Applications include a characterization of roughly similar hyperbolic metrics via mean distortion.

We prove that for a suitable (open) class of open, smoothly bounded domains in the cotangent bundle of a surface of genus $g \geq 2$ any exact symplectomorphism is homotopic to one which is smooth up to the boundary. In particular, such boundaries are not unseen in the sense of Eliashberg–Hofer. The contact boundaries of these domains have Anosov Reeb flows. The methods employed include the properties of such flows in three dimensions, and analysis of two-dimensional transverse flow maps along the lines of Thurston, and thus appear restricted to the case of three-dimensional contact boundaries.

We show that the twisted group $C^{\ast }$ -algebra associated with a discrete FC-hypercentral group is simple (respectively, has a unique tracial state) if and only if Kleppner’s condition is satisfied. This generalizes a result of Packer for countable nilpotent groups. We also consider a larger class of groups, for which we can show that the corresponding reduced twisted group $C^{\ast }$ -algebras have a unique tracial state if and only if Kleppner’s condition holds.

We show that a germ of a holomorphic one-dimensional foliation at a singularity in a space of dimension two admits a holomorphic first integral if and only if there are infinitely many closed leaves and a finite number of separatrices, with each separatrix having linearizable holonomy. Indeed, if there are infinitely many closed leaves and the set of separatrices is finite, then the foliation admits either a holomorphic first integral or a formal simple integrating factor of Darboux type.

We provide a new criterion for the existence of a global cross section to a volume-preserving Anosov flow. The criterion is expressed in terms of expansion and contraction rates of the flow and generalizes known results of this type.

Using an idea of Doug Lind, we give a lower bound for the Perron–Frobenius degree of a Perron number that is not totally real, in terms of the layout of its Galois conjugates in the complex plane. As an application, we prove that there are cubic Perron numbers whose Perron–Frobenius degrees are arbitrary large, a result known to Lind, McMullen and Thurston. A similar result is proved for bi-Perron numbers.

Consider an irrational rotation of a unit circle and a real continuous function. A point is declared ‘maximizing’ if the growth of the ergodic sums at this point is maximal up to an additive constant. In the case of two-sided ergodic sums, the existence of a maximizing point for a continuous function implies that it is the coboundary of a continuous function. In contrast, we build, for the ‘usual’ one-sided ergodic sums, examples in Hölder or smooth classes, indicating that all kinds of behaviour of the function with respect to the dynamical system are possible. We also show that generic continuous functions are without maximizing points, not only for rotations, but also for the transformation 2x mod 1. For this latter transformation, it is known that any Hölder continuous function has a maximizing point.

In this paper we consider a quasi-linear approximation for the 3D Euler system as well as for the Navier–Stokes system in our previous paper Moscow Math. J.1(3) (2001), 381–388). We define finite-dimensional versions of the approximation and study properties of their solutions.

We consider Anosov flows in 3-manifolds. Suppose that there is a rank-two free abelian subgroup of the fundamental group of the manifold, so that none of its elements can be represented by a closed orbit of the flow. We then show that the flow is topologically conjugate to a suspension of an Anosov diffeomorphism. As a consequence we prove that if $T$ is an incompressible torus so that no loop in $T$ is freely homotopic to a closed orbit of the flow, then $T$ is isotopic to a transverse torus. Finally, we show that if $T$ is an incompressible torus transverse to the stable foliation, then either there is a closed leaf in the induced foliation in $T$, or the flow is topologically conjugate to a suspension Anosov flow.

A smooth $\mathbb{Z}^p$-action on a closed orientable manifold M is cohomologically rigid (CR) if the cohomology of the induced $\mathbb{Z}^p$-module on the smooth functions on M is isomorphic to the real cohomology of the torus Tp. We prove some general properties of a smooth $\mathbb{Z}^p$-action whose first cohomology is isomorphic to Rp. When the manifold is a low-dimensional torus Tq, $1 \le q \le 2$, we prove that any minimal (all orbits are dense) smooth $\mathbb{Z}^p$-action on Tq whose first cohomology is isomorphic to Rp is $C^\infty$ conjugate to an affine $\mathbb{Z}^p$-action. As a corollary we show that the CR $\mathbb{Z}^p$-actions on Tq, $1 \le q \le 2$, are smooth conjugations of affine CR actions.

We consider ℝn skew-products of a class of hyperbolic dynamical systems. It was proved by Niţică and Pollicott [Transitivity of Euclidean extensions of Anosov diffeomorphisms. Ergod. Th. & Dynam. Sys. 25 (2005), 257–269] that for an Anosov diffeomorphism ϕ of an infranilmanifold Λ there is (subject to avoiding natural obstructions) an open and dense set f:Λ→ℝN for which the skew-product ϕf(x,v)=(ϕ(x),v+f(x)) on Λ×ℝN has a dense orbit. We prove a similar result in the context of an Axiom A hyperbolic flow on an attractor.

In this paper we consider symplectic and volume-preserving diffeomorphisms of compact manifolds, and prove that, C^{r} generically, 1\leq r\leq\infty, the invariant manifolds of hyperbolic periodic points contain a dense subset of recurrent points. As a consequence, each of these invariant manifolds is contained in its omega limit set, and its closure is a chain transitive set.

We consider the entropy of systems of random transformations, where the transformations are chosen from a set of generators of a ${\Bbb Z}^d $ action. We show that the classical definition gives unsatisfactory entropy results in the higher-dimensional case, i.e. when $d \geq 2$. We propose a definition of the entropy for random group actions which agrees with the classical definition in the one-dimensional case, and which gives satisfactory results in higher dimensions. This definition is based on the fibre entropy of a certain skew product. We identify the entropy by an explicit formula which makes it possible to compute the entropy in certain cases.

We consider continuous cocycles arising from CMV and Jacobi matrices. Assuming that the Verblunsky and Jacobi coefficients arise from generalized skew-shifts, we prove that uniform hyperbolicity of the associated cocycles is $C^0$ -dense. This implies that the associated CMV and Jacobi matrices have a Cantor spectrum for a generic continuous sampling map.

In this article, we provide sufficient conditions on a self-similar interval exchange map, whose renormalization matrix has complex eigenvalues of modulus greater than one, for the existence of affine interval exchange maps with wandering intervals that are semi-conjugate with it. These conditions are based on the algebraic properties of the complex eigenvalues and the complex fractals built from the natural substitution emerging from self-similarity. We show that the cubic Arnoux–Yoccoz interval exchange map satisfies these conditions.

We are interested in attracting sets of $\mathbb{P}^{k}(\mathbb{C})$ which are of small topological degree and of codimension one. We first show that there exists a large family of examples. Then we study their ergodic and pluripotential theoretic properties.