492 research outputs found
Testing the martingale difference hypothesis in high dimension
In this paper, we consider testing the martingale difference hypothesis for
high-dimensional time series. Our test is built on the sum of squares of the
element-wise max-norm of the proposed matrix-valued nonlinear dependence
measure at different lags. To conduct the inference, we approximate the null
distribution of our test statistic by Gaussian approximation and provide a
simulation-based approach to generate critical values. The asymptotic behavior
of the test statistic under the alternative is also studied. Our approach is
nonparametric as the null hypothesis only assumes the time series concerned is
martingale difference without specifying any parametric forms of its
conditional moments. As an advantage of Gaussian approximation, our test is
robust to the cross-series dependence of unknown magnitude. To the best of our
knowledge, this is the first valid test for the martingale difference
hypothesis that not only allows for large dimension but also captures nonlinear
serial dependence. The practical usefulness of our test is illustrated via
simulation and a real data analysis. The test is implemented in a user-friendly
R-function
Statistical inference for high-dimensional spectral density matrix
The spectral density matrix is a fundamental object of interest in time
series analysis, and it encodes both contemporary and dynamic linear
relationships between component processes of the multivariate system. In this
paper we develop novel inference procedures for the spectral density matrix in
the high-dimensional setting. Specifically, we introduce a new global testing
procedure to test the nullity of the cross-spectral density for a given set of
frequencies and across pairs of component indices. For the first time, both
Gaussian approximation and parametric bootstrap methodologies are employed to
conduct inference for a high-dimensional parameter formulated in the frequency
domain, and new technical tools are developed to provide asymptotic guarantees
of the size accuracy and power for global testing. We further propose a
multiple testing procedure for simultaneously testing the nullity of the
cross-spectral density at a given set of frequencies. The method is shown to
control the false discovery rate. Both numerical simulations and a real data
illustration demonstrate the usefulness of the proposed testing methods
Artificial Intelligence-based Motion Tracking in Cancer Radiotherapy: A Review
Radiotherapy aims to deliver a prescribed dose to the tumor while sparing
neighboring organs at risk (OARs). Increasingly complex treatment techniques
such as volumetric modulated arc therapy (VMAT), stereotactic radiosurgery
(SRS), stereotactic body radiotherapy (SBRT), and proton therapy have been
developed to deliver doses more precisely to the target. While such
technologies have improved dose delivery, the implementation of intra-fraction
motion management to verify tumor position at the time of treatment has become
increasingly relevant. Recently, artificial intelligence (AI) has demonstrated
great potential for real-time tracking of tumors during treatment. However,
AI-based motion management faces several challenges including bias in training
data, poor transparency, difficult data collection, complex workflows and
quality assurance, and limited sample sizes. This review serves to present the
AI algorithms used for chest, abdomen, and pelvic tumor motion
management/tracking for radiotherapy and provide a literature summary on the
topic. We will also discuss the limitations of these algorithms and propose
potential improvements.Comment: 36 pages, 5 Figures, 4 Table
Revealing the Signal of QCD Phase Transition in Heavy-Ion Collisions
We propose a novel method to construct the Landau thermodynamic potential
directly from the fluctuations measured in heavy-ion collisions. The potential
is capable of revealing the signal of the critical end-point (CEP) and the
first order phase transition (FOPT) of QCD in the system even away from the
phase transition region. With the available experimental data, we show that the
criterion of the FOPT is negative for most of the collision energies which
indicates no signal of FOPT. The data at GeV with
0-5% centrality shows a different behavior and the mean value of the data
satisfies the criterion. However, the uncertainty is still too large to make a
certain conclusion. The higher order fluctuations are also required for
confirming the signal. We emphasize therefore that new measurements with higher
precision for the within 0-5% centrality in the vicinity of
GeV are in demand which may finally reveal the
signal of QCD phase transition.Comment: 7 pages, 4 figure
Noninflammatory Changes of Microglia Are Sufficient to Cause Epilepsy.
Microglia are well known to play a critical role in maintaining brain homeostasis. However, their role in epileptogenesis has yet to be determined. Here, we demonstrate that elevated mTOR signaling in mouse microglia leads to phenotypic changes, including an amoeboid-like morphology, increased proliferation, and robust phagocytosis activity, but without a significant induction of pro-inflammatory cytokines. We further provide evidence that these noninflammatory changes in microglia disrupt homeostasis of the CNS, leading to reduced synapse density, marked microglial infiltration into hippocampal pyramidal layers, moderate neuronal degeneration, and massive proliferation of astrocytes. Moreover, the mice thus affected develop severe early-onset spontaneous recurrent seizures (SRSs). Therefore, we have revealed an epileptogenic mechanism that is independent of the microglial inflammatory response. Our data suggest that microglia could be an opportune target for epilepsy prevention
Magnetic field-modulated exciton generation in organic semiconductors: an intermolecular quantum correlation effect
Magnetoelectroluminescence (MEL) of organic semiconductor has been
experimentally tuned by adopting blended emitting layer consisting of both hole
and electron transporting materials. A theoretical model considering
intermolecular quantum correlation is proposed to demonstrate two fundamental
issues: (1) two mechanisms, spin scattering and spin mixing, dominate the two
different steps respectively in the process of the magnetic field modulated
generation of exciton; (2) the hopping rate of carriers determines the
intensity of MEL. Calculation successfully predicts the increase of singlet
excitons in low field with little change of triplet exciton population.Comment: 16 pages, 4 figure
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