15 research outputs found
On the almost reducibility conjecture
Avila's Almost Reducibility Conjecture (ARC) is a powerful statement linking
purely analytic and dynamical properties of analytic one frequency
cocycles. It is also a fundamental tool in the study of
spectral theory of analytic one-frequency Schr\"odinger operators, with many
striking consequencies, allowing to give a detailed characterization of the
subcritical region. Here we give a proof, completely different from Avilas, for
the important case of Schr\"odinger cocycles and non-exponentially approximated
frequencies, allowing, in particular, to obtain all the desired spectral
consequences.Comment: 20 page
The H\"older continuity of Lyapunov exponents for a class of Cos-type quasiperiodic Schr\"odinger cocycles
In this paper we obtain exact -H\"older continuity of the
Lyapunov exponents for quasi-periodic Sch\"odinger cocycles with cos-type
potentials, large coupling constants, and fixed Diophantine frequency.
Moreover, we prove the locally Lipschitz continuity of the Lyapunov exponent
for a full measure spectral set. Furthermore, for any given between
to , we can find some energy on the spectrum and on which
Lyapunov exponent is exactly -H\"older continuous.Comment: 88 Page
Robust Source-Free Domain Adaptation for Fundus Image Segmentation
Unsupervised Domain Adaptation (UDA) is a learning technique that transfers
knowledge learned in the source domain from labelled training data to the
target domain with only unlabelled data. It is of significant importance to
medical image segmentation because of the usual lack of labelled training data.
Although extensive efforts have been made to optimize UDA techniques to improve
the accuracy of segmentation models in the target domain, few studies have
addressed the robustness of these models under UDA. In this study, we propose a
two-stage training strategy for robust domain adaptation. In the source
training stage, we utilize adversarial sample augmentation to enhance the
robustness and generalization capability of the source model. And in the target
training stage, we propose a novel robust pseudo-label and pseudo-boundary
(PLPB) method, which effectively utilizes unlabeled target data to generate
pseudo labels and pseudo boundaries that enable model self-adaptation without
requiring source data. Extensive experimental results on cross-domain fundus
image segmentation confirm the effectiveness and versatility of our method.
Source code of this study is openly accessible at
https://github.com/LinGrayy/PLPB.Comment: 10 pages, WACV202
Hofstadter butterflies and metal/insulator transitions for moir\'e heterostructures
We consider a tight-binding model recently introduced by Timmel and Mele for
strained moir\'e heterostructures. We consider two honeycomb lattices to which
layer antisymmetric shear strain is applied to periodically modulate the
tunneling between the lattices in one distinguished direction. This effectively
reduces the model to one spatial dimension and makes it amenable to the theory
of matrix-valued quasi-periodic operators. We then study the transport and
spectral properties of this system, explaining the appearance of a
Hofstadter-type butterfly. For sufficiently incommensurable moir\'e length and
strong coupling between the lattices this leads to the occurrence of
localization phenomena
Kotani theory, Puig's argument, and stability of The Ten Martini Problem
We solve the ten martini problem (Cantor spectrum with no condition on
irrational frequencies, previously only established for the almost Mathieu) for
a large class of one-frequency quasiperiodic operators, including
nonperturbative analytic neighborhoods of several popular explicit families.
The proof is based on the structural analysis of dual cocycles as introduced in
[35]. As a part of the proof, we develop several general ingredients of
independent interest: Kotani theory, for a class of finite-range operators over
general minimal underlying dynamics, making the first step towards and
providing a partial solution of the Kotani-Simon problem, simplicity of point
spectrum for the same class, and the all-frequency version of Puig's argument.Comment: 59 page
Multiplicative Jensen's formula and quantitative global theory of one-frequency Schr\"odinger operators
We introduce the concept of dual Lyapunov exponents, leading to a
multiplicative version of the classical Jensen's formula for one-frequency
analytic Schr\"odinger cocycles. This formula, in particular, gives a new proof
and a quantitative version of the fundamentals of Avila's global theory
\cite{avila}, fully explaining the behavior of complexified Lyapunov exponent
through the dynamics of the dual cocycle. In particular, concepts of
(sub/super) critical regimes and acceleration are all explained (in a
quantitative way) through the duality approach. This leads to a number of
powerful spectral and physics applicationsComment: 40 page