14,367 research outputs found
Forecasting unstable processes
Previous analysis on forecasting theory either assume knowing the true
parameters or assume the stationarity of the series. Not much are known on the
forecasting theory for nonstationary process with estimated parameters. This
paper investigates the recursive least square forecast for stationary and
nonstationary processes with unit roots. We first prove that the accumulated
forecast mean square error can be decomposed into two components, one of which
arises from estimation uncertainty and the other from the disturbance term. The
former, of the order of , is of second order importance to the latter
term, of the order T. However, since the latter is common for all predictors,
it is the former that determines the property of each predictor. Our theorem
implies that the improvement of forecasting precision is of the order of
when existence of unit root is properly detected and taken into
account. Also, our theorem leads to a new proof of strong consistency of
predictive least squares in model selection and a new test of unit root where
no regression is needed. The simulation results confirm our theoretical
findings. In addition, we find that while mis-specification of AR order and
under-specification of the number of unit root have marginal impact on
forecasting precision, over-specification of the number of unit root strongly
deteriorates the quality of long term forecast. As for the empirical study
using Taiwanese data, the results are mixed. Adaptive forecast and imposing
unit root improve forecast precision for some cases but deteriorate forecasting
precision for other cases.Comment: Published at http://dx.doi.org/10.1214/074921706000000969 in the IMS
Lecture Notes Monograph Series
(http://www.imstat.org/publications/lecnotes.htm) by the Institute of
Mathematical Statistics (http://www.imstat.org
Radial Angular Momentum Transfer and Magnetic Barrier for Short-Type Gamma-Ray Burst Central Engine Activity
Soft extended emission (EE) following initial hard spikes up to 100 seconds
was observed with {\em Swift}/BAT for about half of short-type gamma-ray bursts
(SGRBs). This challenges the conversional central engine models of SGRBs, i.e.,
compact star merger models. In the framework of the black hole-neutron star
merger models, we study the roles of the radial angular momentum transfer in
the disk and the magnetic barrier around the black hole for the activity of
SGRB central engines. We show that the radial angular momentum transfer may
significantly prolong the lifetime of the accretion process and multiple
episodes may be switched by the magnetic barrier. Our numerical calculations
based on the models of the neutrino-dominated accretion flows suggest that the
disk mass is critical for producing the observed EE. In case of the mass being
, our model can reproduce the observed timescale and
luminosity of both the main and EE episodes in a reasonable parameter set. The
predicted luminosity of the EE component is lower than the observed EE with
about one order of magnitude and the timescale is shorter than 20 seconds if
the disk mass being . {\em Swift}/BAT-like instruments may
be not sensitive enough to detect the EE component in this case. We argue that
the EE component would be a probe for merger process and disk formation for
compact star mergers.Comment: 9 pages, 3 figures, accepted for publication in Ap
Imputing unknown competitor marketing activity with a Hidden Markov Chain
We demonstrate on a case study with two competing products at a bank how one
can use a Hidden Markov Chain (HMC) to estimate missing information on a
competitor's marketing activity. The idea is that given time series with sales
volumes for products A and B and marketing expenditures for product A, as well
as suitable predictors of sales for products A and B, we can infer at each
point in time whether it is likely or not that marketing activities took place
for product B. The method is successful in identifying the presence or absence
of marketing activity for product B about 84% of the time. We allude to the
issue of whether, if one can infer marketing activity about product B from
knowledge of marketing activity for product A and of sales volumes of both
products, the reverse might be possible and one might be able to impute
marketing activity for product A from knowledge of that of product B. This
leads to a concept of symmetric imputation of competing marketing activity. The
exposition in this paper aims to be accessible and relevant to practitioners
In-Process Global Interpretation for Graph Learning via Distribution Matching
Graphs neural networks (GNNs) have emerged as a powerful graph learning model
due to their superior capacity in capturing critical graph patterns. To gain
insights about the model mechanism for interpretable graph learning, previous
efforts focus on post-hoc local interpretation by extracting the data pattern
that a pre-trained GNN model uses to make an individual prediction. However,
recent works show that post-hoc methods are highly sensitive to model
initialization and local interpretation can only explain the model prediction
specific to a particular instance. In this work, we address these limitations
by answering an important question that is not yet studied: how to provide
global interpretation of the model training procedure? We formulate this
problem as in-process global interpretation, which targets on distilling
high-level and human-intelligible patterns that dominate the training procedure
of GNNs. We further propose Graph Distribution Matching (GDM) to synthesize
interpretive graphs by matching the distribution of the original and
interpretive graphs in the feature space of the GNN as its training proceeds.
These few interpretive graphs demonstrate the most informative patterns the
model captures during training. Extensive experiments on graph classification
datasets demonstrate multiple advantages of the proposed method, including high
explanation accuracy, time efficiency and the ability to reveal class-relevant
structure.Comment: Under Revie
Exothermic isospin-violating dark matter after SuperCDMS and CDEX
We show that exothermic isospin-violating dark matter (IVDM) can make the
results of the latest CDMS-Si experiment consistent with recent null
experiments, such as XENON10, XENON100, LUX, CDEX, and SuperCDMS, whereas for
the CoGeNT experiment, a strong tension still persists. For CDMS-Si, separate
exothermic dark matter or isospin-violating dark matter cannot fully ameliorate
the tensions among these experiments; the tension disappears only if exothermic
scattering is combined with an isospin-violating effect of f_n/f_p=-0.7. For
such exothermic IVDM to exist, at least a new vector gauge boson (dark photon
or dark Z') that connects SM quarks to Majorana-type DM particles is required.Comment: 12 pages, 6 figure
Poly[[pentaaqua(μ4-pyridine-2,4,6-tricarboxylato)(μ3-pyridine-2,4,6-tricarboxylato)disamarium(III)] monohydrate]
The asymmetric unit of the title compound, {[Sm2(C8H2NO6)2(H2O)5]·H2O}n, contains two independent SmIII ions, two pyridine-2,4,6-tricarboxylate (ptc) ligands, five aqua ligands and one lattice water molecule. One SmIII ion is nine-coordinated by one N and five O atoms from the three ptc ligands and three aqua ligands in a distorted monocapped square antiprismatic geometry, and the other is eight-coordinated by one N and five O atoms from three ptc ligands and two aqua ligands in a 4,4′-bicapped trigonal antiprismatic geometry. The ptc ligands brigde the SmIII ions into a three-dimensional polymeric framework. Extensive O—H⋯O hydrogen bonding is observed in the crystal structure
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