69 research outputs found
Sequential Predation: A Multi-model Study
In many ecosystems food resources are available sequentially. The paper analyses a situation with
two competing prey species both of which are consumed by a common predator species. Within a
season the two prey species are available sequentially, although there may be an overlap. Three modelling
methodologies are applied to this system] discrete dynamical systems (difference equations),
individual-oriented event-driven simulations and cellular automata. The presence of the predator is shown
to have a strong impact on the outcome of the prey species competition. The system of coexisting prey
species changes to a system of founder-controlled competition. It appears that sequential predation can
even have counterintuitive evolutionary consequences for the prey species. The species which appears later
in the season will be more successful in its competition with the early species if it favours the predator;
for example, by a high leaf palatability. Spatial structuring and topological issues are found to play a
crucial role in both the ecological and evolutionary dynamics. The advantages of a multi!model approach
are discussed
Semiparametric theory and empirical processes in causal inference
In this paper we review important aspects of semiparametric theory and
empirical processes that arise in causal inference problems. We begin with a
brief introduction to the general problem of causal inference, and go on to
discuss estimation and inference for causal effects under semiparametric
models, which allow parts of the data-generating process to be unrestricted if
they are not of particular interest (i.e., nuisance functions). These models
are very useful in causal problems because the outcome process is often complex
and difficult to model, and there may only be information available about the
treatment process (at best). Semiparametric theory gives a framework for
benchmarking efficiency and constructing estimators in such settings. In the
second part of the paper we discuss empirical process theory, which provides
powerful tools for understanding the asymptotic behavior of semiparametric
estimators that depend on flexible nonparametric estimators of nuisance
functions. These tools are crucial for incorporating machine learning and other
modern methods into causal inference analyses. We conclude by examining related
extensions and future directions for work in semiparametric causal inference
Photoemission and x-ray absorption spectroscopy study of electron-doped colossal magnetoresistance manganite: La0.7Ce0.3MnO3 film
The electronic structure of La0.7Ce0.3MnO3 (LCeMO) thin film has been
investigated using photoemission spectroscopy (PES) and x-ray absorption
spectroscopy (XAS). The Ce 3d core-level PES and XAS spectra of LCeMO are very
similar to those of CeO2, indicating that Ce ions are far from being trivalent.
A very weak 4f resonance is observed around the Ce 4d 4f absorption edge,
suggesting that the localized Ce 4f states are almost empty in the ground
state. The Mn 2p XAS spectrum reveals the existence of the Mn(2+) multiplet
feature, confirming the Mn(2+)-Mn(3+) mixed-valent states of Mn ions in LCeMO.
The measured Mn 3d PES/XAS spectra for LCeMO agrees reasonably well with the
calculated Mn 3d PDOS using the LSDA+U method. The LSDA+U calculation predicts
a half-metallic ground state for LCeMO.Comment: 7 pages, 7 figure
Black Hole Spin via Continuum Fitting and the Role of Spin in Powering Transient Jets
The spins of ten stellar black holes have been measured using the
continuum-fitting method. These black holes are located in two distinct classes
of X-ray binary systems, one that is persistently X-ray bright and another that
is transient. Both the persistent and transient black holes remain for long
periods in a state where their spectra are dominated by a thermal accretion
disk component. The spin of a black hole of known mass and distance can be
measured by fitting this thermal continuum spectrum to the thin-disk model of
Novikov and Thorne; the key fit parameter is the radius of the inner edge of
the black hole's accretion disk. Strong observational and theoretical evidence
links the inner-disk radius to the radius of the innermost stable circular
orbit, which is trivially related to the dimensionless spin parameter a_* of
the black hole (|a_*| < 1). The ten spins that have so far been measured by
this continuum-fitting method range widely from a_* \approx 0 to a_* > 0.95.
The robustness of the method is demonstrated by the dozens or hundreds of
independent and consistent measurements of spin that have been obtained for
several black holes, and through careful consideration of many sources of
systematic error. Among the results discussed is a dichotomy between the
transient and persistent black holes; the latter have higher spins and larger
masses. Also discussed is recently discovered evidence in the transient sources
for a correlation between the power of ballistic jets and black hole spin.Comment: 30 pages. Accepted for publication in Space Science Reviews. Also to
appear in hard cover in the Space Sciences Series of ISSI "The Physics of
Accretion onto Black Holes" (Springer Publisher). Changes to Sections 5.2,
6.1 and 7.4. Section 7.4 responds to Russell et al. 2013 (MNRAS, 431, 405)
who find no evidence for a correlation between the power of ballistic jets
and black hole spi
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