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Series: CDSNS Colloquium

In 1994, Dumortier,
Roussarie and Rousseau launched a program aiming at proving the
ﬁniteness part of Hilbert’s 16th problem for the quadratic
system. For the program, 121 graphics need to be proved to have ﬁnite
cyclicity. In this presentation, I will show that 4 families of
HH-graphics with a triple nilpotent singularity of saddle or elliptic
type have finite cyclicity. Finishing the proof of the cyclicity of
these 4 families of HH-graphics represents one important step towards
the proof of the finiteness part of Hilbert’s 16th problem for
quadratic systems. This is a joint work with Professor Christiane
Rousseau and Professor Huaiping Zhu.

Series: CDSNS Colloquium

This talk is devoted to quasi-periodic Schrödinger operators beyond theAlmost Mathieu, with more general potentials and interactions. The linksbetween the spectral properties of these operators and the dynamicalproperties of the associated quasi-periodic linear skew-products rule thegame. In particular, we present a Thouless formula and some consequencesof Aubry duality. This is a joint work with Joaquim Puig~

Series: CDSNS Colloquium

We present a KAM-like theorem for the existence of
quasi-periodic tori with a prescribed Diophantine rotation for a discrete
family of dynamical system.
The theorem is stated in an a posteriori format, so it can be used to
validate numerical computations. The method of proof provides an efficient
algorithm for computing quasi-periodic tori.
We also present implementations of the algorithm, illustrating them
throught several examples and observing different mechanisms of breakdown
of qp invariant tori.
This is a joint work with Alex Haro.

Series: CDSNS Colloquium

We present a numerical study of the dynamics of a state-dependent delay
equation with two state dependent delays that are linear in the state. In
particular, we study some of the the dynamical behavior driven by the
existence of two-parameter families of invariant tori. A formal normal form
analysis predicts the existence of torus bifurcations and the appearance of
a two parameter family of stable invariant tori. We investigate the
dynamics on the torus thought a Poincaré section. We find some boundaries
of Arnold tongues and indications of loss of normal hyperbolicity for this
stable family. This is joint work with A. R. Humphries and B. Krauskopf.

Series: CDSNS Colloquium

(Joint work with A. Avila and S. Jitomirskaya). An analytic, complex, one-frequency cocycle is given by a pair $(\alpha,A)$ where $A(x)$ is an analytic and 1-periodic function that maps from the torus $\mathbb(R) / \mathbb(Z)$ to the complex $d\times d$ matrices and $\alpha \in [0,1]$ is a frequency.
The pair is interpreted as the map $(\alpha,A)\,:\, (x,v) \mapsto (x+\alpha), A(x) v$.
Associated to the iterates of this map are (averaged) Lyapunov exponents $L_k(\alpha,A)$ and an Osceledets filtration.
We prove joint-continuity in $(\alpha,A)$ of the Lyapunov exponents at irrational frequencies $\alpha$, give a criterion for domination and prove that for a dense open subset of cocycles, the Osceledets filtration comes from a dominated splitting which is an analogue to the Bochi-Viana Theorem.

Series: CDSNS Colloquium

We consider the semi-linear elliptic PDE driven by the fractional Laplacian:
\begin{equation*}\left\{%\begin{array}{ll} (-\Delta)^s u=f(x,u) & \hbox{in $\Omega$,} \\ u=0 & \hbox{in $\mathbb{R}^n\backslash\Omega$.} \\\end{array}%
\right.\end{equation*}An $L^{\infty}$ regularity result is given, using De Giorgi-Stampacchia iteration method.By
the Mountain Pass Theorem and some other nonlinear analysis methods,
the existence and multiplicity of non-trivial solutions for the above
equation are established. The validity of the Palais-Smale condition
without Ambrosetti-Rabinowitz condition for non-local elliptic equations
is proved. Two non-trivial solutions are given under some weak
hypotheses. Non-local elliptic equations with concave-convex
nonlinearities are also studied, and existence of at least six solutions
are obtained.
Moreover, a global result of
Ambrosetti-Brezis-Cerami type is given, which shows that the effect of
the parameter $\lambda$ in the nonlinear term changes considerably the
nonexistence, existence and multiplicity of solutions.

Series: CDSNS Colloquium

Synchronization of coupled oscillators, such as grandfather clocks or
metronomes, has been much studied using the approximation of strong
damping in which case the dynamics of each reduces to a phase on a limit
cycle. This gives rise to the famous Kuramoto model. In contrast, when
the oscillators are Hamiltonian both the amplitude and phase of each
oscillator are dynamically important. A model in which all-to-all
coupling is assumed, called the Hamiltonian Mean Field (HMF) model, was
introduced by Ruffo and his colleagues. As for the Kuramoto model, there
is a coupling strength threshold above which an incoherent state loses
stability and the oscillators synchronize.
We study the case when the moments of inertia and coupling strengths of
the oscillators are heterogeneous. We show that finite size fluctuations
can greatly modify the synchronization threshold by inducing
correlations between the momentum and parameters of the rotors. For
unimodal parameter distributions, we find an analytical expression for
the modified critical coupling strength in terms of statistical
properties of the parameter distributions and confirm our results with
numerical simulations. We find numerically that these effects disappear
for strongly bimodal parameter distributions.
*This work is in collaboration with Juan G. Restrepo.

Series: CDSNS Colloquium

The three objects in the title come together in the study of
ergodic properties of geodesic flows on flat surfaces. I will go over how
these three things are intimately related, state some classical results
about the unique ergodicity of translation flows and present new results
which generalize much of the classical theory and also apply to non-compact
(infinite genus) surfaces.

Series: CDSNS Colloquium

There is known a lot of information about classical or standard shadowing. Itis also often called a pseudo-orbit tracing property (POTP). Let M be a closedRiemannian manifold. Diffeomorphism f : M → M is said to have POTPif for a given accuracy any pseudotrajectory with errors small enough can beapproximated (shadowed) by an exact trajectory. Therefore, if one wants to dosome numerical investiagion of the system one would definitely prefer it to haveshadowing property.However, now it is widely accepted that good (qualitatively strong) shad-owing is present only in hyperbolic situations. However it seems that manynonhyperbolic systems still could be well analysed numerically.As a step to resolve this contradiction I introduce some sort of weaker shad-owing. The idea is to restrict a set of pseudotrajectories to be shadowed. Onecan consider only pseudotrajectories that resemble sequences of points generatedby a computer with floating-point arithmetic.I will tell what happens in the (simplified) case of “linear” two-dimensionalsaddle connection. In this case even stochastic versions of classical shadowing(when one tries to ask only for most pseudotrajectories to be shadowed) do notwork. Nevertheless, for “floating-point” pseudotrajectories one can prove somepositive results.There is a dichotomy: either every pseudotrajectory stays close to the un-perturbed trajectory forever if one carefully chooses the dependence betweenthe size of errors and requested accuracy of shadowing, or there is always apseudotrajectory that can not be shadowed.

Series: CDSNS Colloquium

To identify and to explain coupling-induced phase transitions in
Coupled Map Lattices (CML) has been a lingering enigma for about two
decades. In numerical simulations, this phenomenon has always been observed
preceded by a lowering of the Lyapunov dimension, suggesting that the
transition might require changes of linear stability. Yet, recent proofs of
co-existence of several phases in specially designed models work in the
expanding regime where all Lyapunov exponents remain positive.
In this talk, I will consider a family of CML composed by piecewise
expanding individual map, global interaction and finite number N of sites,
in the weak coupling regime where the CML is uniformly expanding.
I will show, mathematically for N=3 and numerically for N>3, that a
transition in the asymptotic dynamics occurs as the coupling strength
increases. The transition breaks the (Milnor) attractor into several
chaotic pieces of positive Lebesgue measure, with distinct empiric
averages. It goes along with various symmetry breaking, quantified by means
of magnetization-type characteristics.
Despite that it only addresses finite-dimensional systems, to some extend,
this result reconciles the previous ones as it shows that loss of
ergodicity/symmetry breaking can occur in basic CML, independently of any
decay in the Lyapunov dimension.