839 research outputs found
A Bayesian Approach to Inverse Quantum Statistics
A nonparametric Bayesian approach is developed to determine quantum
potentials from empirical data for quantum systems at finite temperature. The
approach combines the likelihood model of quantum mechanics with a priori
information over potentials implemented in form of stochastic processes. Its
specific advantages are the possibilities to deal with heterogeneous data and
to express a priori information explicitly, i.e., directly in terms of the
potential of interest. A numerical solution in maximum a posteriori
approximation was feasible for one--dimensional problems. Using correct a
priori information turned out to be essential.Comment: 4 pages, 6 figures, revte
Semiclassical Quantisation Using Diffractive Orbits
Diffraction, in the context of semiclassical mechanics, describes the manner
in which quantum mechanics smooths over discontinuities in the classical
mechanics. An important example is a billiard with sharp corners; its
semiclassical quantisation requires the inclusion of diffractive periodic
orbits in addition to classical periodic orbits. In this paper we construct the
corresponding zeta function and apply it to a scattering problem which has only
diffractive periodic orbits. We find that the resonances are accurately given
by the zeros of the diffractive zeta function.Comment: Revtex document. Submitted to PRL. Figures available on reques
Chaotic wave functions and exponential convergence of low-lying energy eigenvalues
We suggest that low-lying eigenvalues of realistic quantum many-body
hamiltonians, given, as in the nuclear shell model, by large matrices, can be
calculated, instead of the full diagonalization, by the diagonalization of
small truncated matrices with the exponential extrapolation of the results. We
show numerical data confirming this conjecture. We argue that the exponential
convergence in an appropriate basis may be a generic feature of complicated
("chaotic") systems where the wave functions are localized in this basis.Comment: 4 figure
Likelihood Geometry
We study the critical points of monomial functions over an algebraic subset
of the probability simplex. The number of critical points on the Zariski
closure is a topological invariant of that embedded projective variety, known
as its maximum likelihood degree. We present an introduction to this theory and
its statistical motivations. Many favorite objects from combinatorial algebraic
geometry are featured: toric varieties, A-discriminants, hyperplane
arrangements, Grassmannians, and determinantal varieties. Several new results
are included, especially on the likelihood correspondence and its bidegree.
These notes were written for the second author's lectures at the CIME-CIRM
summer course on Combinatorial Algebraic Geometry at Levico Terme in June 2013.Comment: 45 pages; minor changes and addition
Collisional Damping of Nuclear Collective Vibrations in a Non-Markovian Transport Approach
A detailed derivation of the collisional widths of collective vibrations is
presented in both quantal and semi-classical frameworks by considering the
linearized limits of the extended TDHF and the BUU model with a non-Markovian
binary collision term. Damping widths of giant dipole and giant quadrupole
excitations are calculated by employing an effective Skyrme force, and the
results are compared with GDR measurements in Lead and Tin nuclei at finite
temperature.Comment: 23 pages, 6 Figure
Health professionals, their medical interventions and uncertainty : a study focusing on women at midlife
Health professionals face a tension between focusing on the individual and attending to health issues for the population as a whole. This tension is intrinsic to medicine and gives rise to medical uncertainty, which here is explored through accounts of three medical interventions focused on women at midlife: breast screening, hormone replacement therapy and bone densitometry. The accounts come from interviews with UK health professionals using these medical interventions in their daily work. Drawing on the analysis of Fox [(2002). Health and Healing: The public/private divide (pp. 236–253). London: Routledge] we distinguish three aspects of medical uncertainty and explore each one of them in relation to one of the interventions. First is uncertainty about the balance between the individual and distributive ethic of medicine, explored in relation to breast screening. Second is the dilemma faced by health professionals when using medicial evidence generated through studies of populations and applying this to individuals. We explore this dilemma for hormone replacement therapy. Thirdly there is uncertainty because of the lack of a conceptual framework for understanding how new micro knowledge, such as human genetic information, can be combined with knowledge of other biological and social dimensions of health. The accounts from the bone denistometry clinic indicate the beginnings of an understanding of the need for such a framework, which would acknowledge complexity, recognising that factors from many different levels of analysis, from heredity through to social factors, interact with each other and influence the individual and their health. However, our analysis suggests biomedicine continues to be dominated by an individualised, context free, concept of health and health risk with individuals alone responsible for their own health and for the health of the population. This may continue to dominate how we perceive responsibilities for health until we establish a conceptual framework that recognises the complex interaction of many factors at macro and micro level affecting health
Binary Models for Marginal Independence
Log-linear models are a classical tool for the analysis of contingency
tables. In particular, the subclass of graphical log-linear models provides a
general framework for modelling conditional independences. However, with the
exception of special structures, marginal independence hypotheses cannot be
accommodated by these traditional models. Focusing on binary variables, we
present a model class that provides a framework for modelling marginal
independences in contingency tables. The approach taken is graphical and draws
on analogies to multivariate Gaussian models for marginal independence. For the
graphical model representation we use bi-directed graphs, which are in the
tradition of path diagrams. We show how the models can be parameterized in a
simple fashion, and how maximum likelihood estimation can be performed using a
version of the Iterated Conditional Fitting algorithm. Finally we consider
combining these models with symmetry restrictions
Uniform random generation of large acyclic digraphs
Directed acyclic graphs are the basic representation of the structure
underlying Bayesian networks, which represent multivariate probability
distributions. In many practical applications, such as the reverse engineering
of gene regulatory networks, not only the estimation of model parameters but
the reconstruction of the structure itself is of great interest. As well as for
the assessment of different structure learning algorithms in simulation
studies, a uniform sample from the space of directed acyclic graphs is required
to evaluate the prevalence of certain structural features. Here we analyse how
to sample acyclic digraphs uniformly at random through recursive enumeration,
an approach previously thought too computationally involved. Based on
complexity considerations, we discuss in particular how the enumeration
directly provides an exact method, which avoids the convergence issues of the
alternative Markov chain methods and is actually computationally much faster.
The limiting behaviour of the distribution of acyclic digraphs then allows us
to sample arbitrarily large graphs. Building on the ideas of recursive
enumeration based sampling we also introduce a novel hybrid Markov chain with
much faster convergence than current alternatives while still being easy to
adapt to various restrictions. Finally we discuss how to include such
restrictions in the combinatorial enumeration and the new hybrid Markov chain
method for efficient uniform sampling of the corresponding graphs.Comment: 15 pages, 2 figures. To appear in Statistics and Computin
The selectivity, voltage-dependence and acid sensitivity of the tandem pore potassium channel TASK-1 : contributions of the pore domains
We have investigated the contribution to ionic
selectivity of residues in the selectivity filter and pore
helices of the P1 and P2 domains in the acid sensitive
potassium channel TASK-1. We used site directed mutagenesis
and electrophysiological studies, assisted by structural
models built through computational methods. We have
measured selectivity in channels expressed in Xenopus
oocytes, using voltage clamp to measure shifts in reversal
potential and current amplitudes when Rb+ or Na+ replaced
extracellular K+. Both P1 and P2 contribute to selectivity,
and most mutations, including mutation of residues in the
triplets GYG and GFG in P1 and P2, made channels nonselective.
We interpret the effects of these—and of other
mutations—in terms of the way the pore is likely to be
stabilised structurally. We show also that residues in the
outer pore mouth contribute to selectivity in TASK-1.
Mutations resulting in loss of selectivity (e.g. I94S, G95A)
were associated with slowing of the response of channels to
depolarisation. More important physiologically, pH sensitivity
is also lost or altered by such mutations. Mutations
that retained selectivity (e.g. I94L, I94V) also retained their
response to acidification. It is likely that responses both to
voltage and pH changes involve gating at the selectivity filter
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