We present a comprehensive mechanism for the emergence of strange attractors in a two-parametric family of differential equations acting on a three-dimensional sphere. When both parameters are zero, its flow exhibits an attracting heteroclinic network (Bykov network) made by two 1-dimensional and one 2-dimensional separatrices between two hyperbolic saddles-foci with different Morse indices.

This is the Dynamical Systems Group closing activity of 2019. It consists of three
40min talks, described below:

Ana Rodrigues (Exeter): Recent advances on fair measures and fair entropy

We study a topological entropy of a finitely generated semigroup G of continuous selfmaps defined on a compact metric space X. The topological entropy coincides with the limit of upper capacities of dynamically defined Caratheodory structures on X depending on G.

A well-known result of Bowen says that the periodic orbits of a hyperbolic flow become, on average, equidistributed with respect to the measure of maximal entropy as the periods tend to infinity. By introducing weightings, Parry showed that other Gibbs states could arise as limits in this way. We will discuss what happens if we restrict to Anosov flows are only consider periodic orbits which are trivial in homology.

In this talk I will discuss how to compute the entropy following backwards trajectories in a way that at each step every preimage can be chosen with equal probability introducing fair measure and fair entropy (joint with M. Misiurewicz). I will discuss some advances on the study of fair entropy for non-invertible interval maps under the framework of thermodynamic formalism, showing that the fair measure is usually an equilibrium state (joint with Y. Zhang). I will then talk about some recent results (joint with S. and Z.

We present a deterministic discrete dynamical system which is used to classify and simulate complex irregular motion, in two or three dimension. The dynamical system is defined by a two-parameter family of bimodal interval maps which give directly the displacements through iteration. A trajectory is composed of patches of linear motions, through the plane, intertwined by changes of direction. The characterization of the movements is obtained from the topological classification of the interval map family.

Let f and g be piecewise smooth interval maps, with critical-singular sets, and A a cycle of intervals for f . We prove that A is a topological chaotic attractor if, and only if, A is a metric chaotic attractor. Let h|A be a topological conjugacy between f and g. We prove that, if h is differentiable at a single point p of the visiting set V , with non zero derivative, then h is smooth in A. Furthermore, the visiting set V is a residual set of A and, if the sets Cf and Cg are critical then V has µ full measure, for every expanding measure µ, with supp µ = A

In this talk, we discuss contributions to the thermodynamic formalism of partially hyperbolic attractors for non-singular endomorphisms. We consider a class of local diffeomorphisms admitting an invariant stable bundle and a positively invariant cone field with non-uniform cone expansion at a positive Lebesgue measure set of points. We present how to construct SRB measures for this class of maps and prove estimates for the measure of dynamical balls with respect to the volume measure and the SRB measure.

For C^k quasi-periodic Schrodinger operators in the local perturbative regime, we prove that the length of the corresponding spectral gap has a polynomial decay upper bound w.r.t. its label. This is based on a refined quantitative reducibility theorem for C^k quasi-periodic SL(2,R) cocycles. As an application, we are able to show the homogeneity of the spectrum.

We deal with germs of nvolutions: homeomorphisms $R$ that are their own inverse. We are interested in germs of homeomorphisms $F$ that are $R$-reversible: $F \circ R = R \circ F^{-1}$. We establish that this condition implies that, in general, both the family of reversing symmetries and the group of symmetries of a given germ are not finite, in contrast with continuous-time dynamics, where typically there are finitely many reversing symmetries.