692 research outputs found
Is the growth of the Scottish economy the 'first priority' for public spending in Scotland
The Scottish Executive, notably the First Minister, has frequently stated that the growth and development of the Scottish Economy is the first priority of government in Scotland
Pattern formation in individual-based systems with time-varying parameters
We study the patterns generated in finite-time sweeps across
symmetry-breaking bifurcations in individual-based models. Similar to the
well-known Kibble-Zurek scenario of defect formation, large-scale patterns are
generated when model parameters are varied slowly, whereas fast sweeps produce
a large number of small domains. The symmetry breaking is triggered by
intrinsic noise, originating from the discrete dynamics at the micro-level.
Based on a linear-noise approximation, we calculate the characteristic length
scale of these patterns. We demonstrate the applicability of this approach in a
simple model of opinion dynamics, a model in evolutionary game theory with a
time-dependent fitness structure, and a model of cell differentiation. Our
theoretical estimates are confirmed in simulations. In further numerical work,
we observe a similar phenomenon when the symmetry-breaking bifurcation is
triggered by population growth.Comment: 16 pages, 9 figures. Published version. Corrected missing appendix
link from previous versio
Stochastic tunneling and metastable states during the somatic evolution of cancer
Tumors initiate when a population of proliferating cells accumulates a
certain number and type of genetic and/or epigenetic alterations. The
population dynamics of such sequential acquisition of (epi)genetic alterations
has been the topic of much investigation. The phenomenon of stochastic
tunneling, where an intermediate mutant in a sequence does not reach fixation
in a population before generating a double mutant, has been studied using a
variety of computational and mathematical methods. However, the field still
lacks a comprehensive analytical description since theoretical predictions of
fixation times are only available for cases in which the second mutant is
advantageous. Here, we study stochastic tunneling in a Moran model. Analyzing
the deterministic dynamics of large populations we systematically identify the
parameter regimes captured by existing approaches. Our analysis also reveals
fitness landscapes and mutation rates for which finite populations are found in
long-lived metastable states. These are landscapes in which the final mutant is
not the most advantageous in the sequence, and resulting metastable states are
a consequence of a mutation-selection balance. The escape from these states is
driven by intrinsic noise, and their location affects the probability of
tunneling. Existing methods no longer apply. In these regimes it is the escape
from the metastable states that is the key bottleneck; fixation is no longer
limited by the emergence of a successful mutant lineage. We used the so-called
Wentzel-Kramers-Brillouin method to compute fixation times in these parameter
regimes, successfully validated by stochastic simulations. Our work fills a gap
left by previous approaches and provides a more comprehensive description of
the acquisition of multiple mutations in populations of somatic cells.Comment: 33 pages, 7 figure
When the mean is not enough: Calculating fixation time distributions in birth-death processes
Studies of fixation dynamics in Markov processes predominantly focus on the
mean time to absorption. This may be inadequate if the distribution is broad
and skewed. We compute the distribution of fixation times in one-step
birth-death processes with two absorbing states. These are expressed in terms
of the spectrum of the process, and we provide different representations as
forward-only processes in eigenspace. These allow efficient sampling of
fixation time distributions. As an application we study evolutionary game
dynamics, where invading mutants can reach fixation or go extinct. We also
highlight the median fixation time as a possible analog of mixing times in
systems with small mutation rates and no absorbing states, whereas the mean
fixation time has no such interpretation.Comment: Published in PRE. 14 pages, 6 figure
Nuclear recoil energy scale in liquid xenon with application to the direct detection of dark matter
We show for the first time that the quenching of electronic excitation from
nuclear recoils in liquid xenon is well-described by Lindhard theory, if the
nuclear recoil energy is reconstructed using the combined (scintillation and
ionization) energy scale proposed by Shutt {\it et al.}. We argue for the
adoption of this perspective in favor of the existing preference for
reconstructing nuclear recoil energy solely from primary scintillation. We show
that signal partitioning into scintillation and ionization is well-described by
the Thomas-Imel box model. We discuss the implications for liquid xenon
detectors aimed at the direct detection of dark matter
Devolution and the economy : a Scottish perspective
In their interesting and challenging chapter John Adams and Peter Robinson assess the consequences for economic development policy of the devolution measures enacted by the UK Labour government post 1997. Their chapter ranges widely over current UK regional disparities, the link between devolution and economic growth, the balance of responsibilities in policy between Whitehall and the devolved administrations, and finally, they raise questions about the developing "quasi-federal" role of Whitehall in regulating or coordinating the new devolved policy landscape. In response, we propose to focus on four issues that we believe are key to understanding the economic consequences of devolution both at the Scottish and UK levels. First, we argue that the view of Scotland's devolutionary experience in economic policy is partial and so does not fully capture the nature and extent of change post 1999. Secondly, we examine the role of devolution in regional economic performance. There is much in their paper on this topic with which we agree but we contend that there are significant omissions in the analysis, which are important for policy choice. Our third section highlights an area not discussed in depth by Adams and Robinson's paper: the funding of the devolution settlement. Here we consider some of the implications of funding arrangements for economic performance and the options for a new funding settlement. Finally, we deal with the difficult issue of co-ordination between the centre and the devolved regions. We contend that co-ordination is largely conspicuous by its absence. Moreover, where coordination is deployed it reflects an inadequate understanding of the extent to which the economies of the regions and devolved territories of the UK are linked
Effects of spin vacancies on magnetic properties of the Kitaev-Heisenberg model
We study the ground state properties of the Kitaev-Heisenberg model in a
magnetic field and explore the evolution of spin correlations in the presence
of non-magnetic vacancies. By means of exact diagonalizations, the phase
diagram without vacancies is determined as a function of the magnetic field and
the ratio between Kitaev and Heisenberg interactions. We show that in the
(antiferromagnetic) stripe ordered phase the static susceptibility and its
anisotropy can be described by a spin canting mechanism. This accounts as well
for the transition to the polarized phase when including quantum fluctuations
perturbatively. Effects of spin vacancies depend sensitively on the type of the
ground state. In the liquid phase, the magnetization pattern around a single
vacancy in a small field is determined, and its spatial anisotropy is related
to that of non-zero further neighbor correlations induced by the field and/or
Heisenberg interactions. In the stripe phase, the joint effect of a vacancy and
a small field breaks the six-fold symmetry of the model and stabilizes a
particular stripe pattern. Similar symmetry-breaking effects occur even at zero
field due to effective interactions between vacancies. This selection mechanism
and intrinsic randomness of vacancy positions may lead to spin-glass behavior.Comment: 13 pages, 10 figure
Low-Temperature Hall Effect in Substituted Sr2RuO4
We report the results of a study of the Hall effect and magnetoresistance in
single crystals of Sr2RuO4 in which Sr^(2+) has been substituted by La^(3+)
(Sr(2-y)La(y)RuO(4)) or Ru^(4+) by Ti^(4+) (Sr(2)Ru(1-x)Ti(x)O(4)). For undoped
Sr2RuO4, the purity is so high that the strong-field Hall coefficient can be
measured for fields above 4 T. The conventional weak-field Hall coefficient as
a function of doping shows a sharp jump and sign change at y ~ 0.01 that is
unrelated to either a sharp change in Fermi-surface topography or a magnetic
instability. The implications of these results are discussed.Comment: 5 pages, 4 figure
Fixation in finite populations evolving in fluctuating environments
The environment in which a population evolves can have a crucial impact on selection. We study evolutionary dynamics in finite populations of fixed size in a changing environment. The population dynamics are driven by birth and death events. The rates of these events may vary in time depending on the state of the environment, which follows an independent Markov process. We develop a general theory for the fixation probability of a mutant in a population of wild-types, and for mean unconditional and conditional fixation times. We apply our theory to evolutionary games for which the payoff structure varies in time. The mutant can exploit the environmental noise; a dynamic environment that switches between two states can lead to a probability of fixation that is higher than in any of the individual environmental states. We provide an intuitive interpretation of this surprising effect. We also investigate stationary distributions when mutations are present in the dynamics. In this regime, we find two approximations of the stationary measure. One works well for rapid switching, the other for slowly fluctuating environments
- …