1,189,994 research outputs found
Quantum discrete Dubrovin equations
The discrete equations of motion for the quantum mappings of KdV type are
given in terms of the Sklyanin variables (which are also known as quantum
separated variables). Both temporal (discrete-time) evolutions and spatial
(along the lattice at a constant time-level) evolutions are considered. In the
classical limit, the temporal equations reduce to the (classical) discrete
Dubrovin equations as given in a previous publication. The reconstruction of
the original dynamical variables in terms of the Sklyanin variables is also
achieved.Comment: 25 page
Learning Temporal Dependence from Time-Series Data with Latent Variables
We consider the setting where a collection of time series, modeled as random
processes, evolve in a causal manner, and one is interested in learning the
graph governing the relationships of these processes. A special case of wide
interest and applicability is the setting where the noise is Gaussian and
relationships are Markov and linear. We study this setting with two additional
features: firstly, each random process has a hidden (latent) state, which we
use to model the internal memory possessed by the variables (similar to hidden
Markov models). Secondly, each variable can depend on its latent memory state
through a random lag (rather than a fixed lag), thus modeling memory recall
with differing lags at distinct times. Under this setting, we develop an
estimator and prove that under a genericity assumption, the parameters of the
model can be learned consistently. We also propose a practical adaption of this
estimator, which demonstrates significant performance gains in both synthetic
and real-world datasets
Temporal and spatial variation of limnological variables and biomass of different macrophyte species in a Neotropical reservoir (SĂŁo Paulo - Brazil)
Aim: This study reports an investigation of limnological characteristics
and aquatic macrophyte occurrence in a neotropical reservoir in order to assess the
spatio-temporal variation of water and sediment variables and their influence on plant
distribution. Methods: Macrophytes, water and sediment samples were collected from
a Brazilian reservoir in different seasons from four main arms of the reservoir. In total
sixteen water-sediment variables were analyzed including N:P ratio and Trophic State
Index. The plants were collected using a quadrat sampling procedure and the dry weight
per sample was measured. MANOVA was performed to evaluate spatial and temporal
variation of environmental variables as well as seasonal biomass differences. To assess
the relationship among environmental variables and macrophytes an ordination analysis
(using Canonical Correspondence Analysis: CCA) was carried out. Results: The spatial
and temporal variation of limnological variables generated a heterogeneous system which
supports the presence of different species of macrophyte. pH, dissolved oxygen and
sediment composition were important predictors of Polygonum lapathifolium occurrence
while nutrients were associated with Eichhornia crassipes and Pistia stratiotes. Inorganic
substances were related to biomass variation of Eichhornia azurea and Myriophyllum
aquaticum. Conclusions: The spatial variation of the environmental variables has caused
heterogeneity in the reservoir and it may support the occurrence of different species of
macrophyte. Limnological variables highlighted in CCA are important to predict the
species occurrence and their control in the study area
Self-adjoint Lyapunov variables, temporal ordering and irreversible representations of Schroedinger evolution
In non relativistic quantum mechanics time enters as a parameter in the
Schroedinger equation. However, there are various situations where the need
arises to view time as a dynamical variable. In this paper we consider the
dynamical role of time through the construction of a Lyapunov variable - i.e.,
a self-adjoint quantum observable whose expectation value varies monotonically
as time increases. It is shown, in a constructive way, that a certain class of
models admit a Lyapunov variable and that the existence of a Lyapunov variable
implies the existence of a transformation mapping the original quantum
mechanical problem to an equivalent irreversible representation. In addition,
it is proved that in the irreversible representation there exists a natural
time ordering observable splitting the Hilbert space at each t>0 into past and
future subspaces.Comment: Accepted for publication in JMP. Supercedes arXiv:0710.3604.
Discussion expanded to include the case of Hamiltonians with an infinitely
degenerate spectru
Multivariate Covariance Generalized Linear Models
We propose a general framework for non-normal multivariate data analysis
called multivariate covariance generalized linear models (McGLMs), designed to
handle multivariate response variables, along with a wide range of temporal and
spatial correlation structures defined in terms of a covariance link function
combined with a matrix linear predictor involving known matrices. The method is
motivated by three data examples that are not easily handled by existing
methods. The first example concerns multivariate count data, the second
involves response variables of mixed types, combined with repeated measures and
longitudinal structures, and the third involves a spatio-temporal analysis of
rainfall data. The models take non-normality into account in the conventional
way by means of a variance function, and the mean structure is modelled by
means of a link function and a linear predictor. The models are fitted using an
efficient Newton scoring algorithm based on quasi-likelihood and Pearson
estimating functions, using only second-moment assumptions. This provides a
unified approach to a wide variety of different types of response variables and
covariance structures, including multivariate extensions of repeated measures,
time series, longitudinal, spatial and spatio-temporal structures.Comment: 21 pages, 5 figure
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