1,367 research outputs found
Solving the Bethe-Salpeter equation for bound states of scalar theories in Minkowski space
We apply the perturbation theory integral representation (PTIR) to solve for
the bound state Bethe-Salpeter (BS) vertex for an arbitrary scattering kernel,
without the need for any Wick rotation. The results derived are applicable to
any scalar field theory (without derivative coupling). It is shown that solving
directly for the BS vertex, rather than the BS amplitude, has several major
advantages, notably its relative simplicity and superior numerical accuracy. In
order to illustrate the generality of the approach we obtain numerical
solutions using this formalism for a number of scattering kernels, including
cases where the Wick rotation is not possible.Comment: 28 pages of LaTeX, uses psfig.sty with 5 figures. Also available via
WWW at
http://www.physics.adelaide.edu.au/theory/papers/ADP-97-10.T248-abs.html or
via anonymous ftp at
ftp://bragg.physics.adelaide.edu.au/pub/theory/ADP-97-10.T248.ps A number of
(crucial) typographical errors in Appendix C corrected. To be published in
Phys. Rev. D, October 199
First-Order Logic Theorem Proving and Model Building via Approximation and Instantiation
In this paper we consider first-order logic theorem proving and model
building via approximation and instantiation. Given a clause set we propose its
approximation into a simplified clause set where satisfiability is decidable.
The approximation extends the signature and preserves unsatisfiability: if the
simplified clause set is satisfiable in some model, so is the original clause
set in the same model interpreted in the original signature. A refutation
generated by a decision procedure on the simplified clause set can then either
be lifted to a refutation in the original clause set, or it guides a refinement
excluding the previously found unliftable refutation. This way the approach is
refutationally complete. We do not step-wise lift refutations but conflicting
cores, finite unsatisfiable clause sets representing at least one refutation.
The approach is dual to many existing approaches in the literature because our
approximation preserves unsatisfiability
Nonperturbative study of generalized ladder graphs in a \phi^2\chi theory
The Feynman-Schwinger representation is used to construct scalar-scalar bound
states for the set of all ladder and crossed-ladder graphs in a \phi^2\chi
theory in (3+1) dimensions. The results are compared to those of the usual
Bethe-Salpeter equation in the ladder approximation and of several
quasi-potential equations. Particularly for large couplings, the ladder
predictions are seen to underestimate the binding energy significantly as
compared to the generalized ladder case, whereas the solutions of the
quasi-potential equations provide a better correspondence. Results for the
calculated bound state wave functions are also presented.Comment: 5 pages revtex, 3 Postscripts figures, uses epsf.sty, accepted for
publication in Physical Review Letter
Study of relativistic bound states for scalar theories in Bethe-Salpeter and Dyson-Schwinger formalism
The Bethe-Salpeter equation for Wick-Cutkosky like models is solved in
dressed ladder approximation. The bare vertex truncation of the Dyson-Schwinger
equations for propagators is combined with the dressed ladder Bethe-Salpeter
equation for the scalar S-wave bound state amplitudes. With the help of
spectral representation the results are obtained directly in Minkowski space.
We give a new analytic formula for the resulting equation simplifying the
numerical treatment. The bare ladder approximation of Bethe-Salpeter equation
is compared with the one with dressed ladder. The elastic electromagnetic form
factors is calculated within the relativistic impulse approximation.Comment: 30 pages, 10 figures, accepted for publication in Phys. Rev.
Questionnaires to Assess Facilitators and Barriers of Early Mobilization in Critically Ill Patients; Which One to Choose? A Systematic Review
Implementing and performing early mobilization is a complex process requiring multidisciplinary input and cooperation. To gain insight in its facilitators and barriers, various surveys have been developed. A systematic review was conducted, to identify the psychometric properties, feasibility and suitability of questionnaires to assess facilitators and barriers of early mobilization in critically ill patients. Data were extracted regarding a.o. definition of early mobilization, development, psychometric properties, content and themes, question format. The search identified 537 publications of which 13 unique questionnaires were included. The questionnaires showed wide variation in extensiveness of development. Only six questionnaires actually assessed validity and reliability. Which questionnaire to choose depends on the aim of its use, required level of detail and specifics of the ICU, though three questionnaires were recommended as their definition of early mobilization covered a broad range of activities, including nursing related mobility activities. International consensus on what constitutes early mobilization is desirable
A CDCL-style calculus for solving non-linear constraints
In this paper we propose a novel approach for checking satisfiability of
non-linear constraints over the reals, called ksmt. The procedure is based on
conflict resolution in CDCL style calculus, using a composition of symbolical
and numerical methods. To deal with the non-linear components in case of
conflicts we use numerically constructed restricted linearisations. This
approach covers a large number of computable non-linear real functions such as
polynomials, rational or trigonometrical functions and beyond. A prototypical
implementation has been evaluated on several non-linear SMT-LIB examples and
the results have been compared with state-of-the-art SMT solvers.Comment: 17 pages, 3 figures; accepted at FroCoS 2019; software available at
<http://informatik.uni-trier.de/~brausse/ksmt/
Episkeletozoans and bioerosional ichnotaxa on isolated bones of Late Cretaceous mosasaurs and cheloniid turtles from the Maastricht area, the Netherlands
Isolated bones of three taxa of marine reptiles (Mosasaurus hoffmannii Mantell, Plioplatecarpus marshi Dollo and Allopleuron
hofmanni (Gray)) from various levels within the Maastricht Formation (upper Maastrichtian) at the former
ENCI-HeidelbergCement Group quarry (Maastricht, the Netherlands) exhibit bioerosional traces and encrustation.
Episkeletozoans include dimyid, ostreid and monopleurid bivalves, at least three species of cheilostome and cyclostome
bryozoans and two adnate calcareous foraminifera. The bones show biting traces (Gnathichnus pentax Bromley, Linichnus
cf. serratus Jacobsen & Bromley and Machichnus isp.), as well as borings. The latter may be referred to Karethraichnus
lakkos Zonneveld, Bartels, Gunnell & McHugh, which is here considered to be a junior synonym of Gastrochaenolites isp
Relativistic bound-state equations in three dimensions
Firstly, a systematic procedure is derived for obtaining three-dimensional
bound-state equations from four-dimensional ones. Unlike ``quasi-potential
approaches'' this procedure does not involve the use of delta-function
constraints on the relative four-momentum. In the absence of negative-energy
states, the kernels of the three-dimensional equations derived by this
technique may be represented as sums of time-ordered perturbation theory
diagrams. Consequently, such equations have two major advantages over
quasi-potential equations: they may easily be written down in any Lorentz
frame, and they include the meson-retardation effects present in the original
four-dimensional equation. Secondly, a simple four-dimensional equation with
the correct one-body limit is obtained by a reorganization of the generalized
ladder Bethe-Salpeter kernel. Thirdly, our approach to deriving
three-dimensional equations is applied to this four-dimensional equation, thus
yielding a retarded interaction for use in the three-dimensional bound-state
equation of Wallace and Mandelzweig. The resulting three-dimensional equation
has the correct one-body limit and may be systematically improved upon. The
quality of the three-dimensional equation, and our general technique for
deriving such equations, is then tested by calculating bound-state properties
in a scalar field theory using six different bound-state equations. It is found
that equations obtained using the method espoused here approximate the wave
functions obtained from their parent four-dimensional equations significantly
better than the corresponding quasi-potential equations do.Comment: 28 pages, RevTeX, 6 figures attached as postscript files. Accepted
for publication in Phys. Rev. C. Minor changes from original version do not
affect argument or conclusion
Solitosynthesis of Q-balls
We study the formation of Q-balls in the early universe, concentrating on
potentials with a cubic or quartic attractive interaction. Large Q-balls can
form via solitosynthesis, a process of gradual charge accretion, provided some
primordial charge assymetry and initial ``seed'' Q-balls exist. We find that
such seeds are possible in theories in which the attractive interaction is of
the form , with a light ``Higgs'' mass. Condensate formation
and fragmentation is only possible for masses in the sub-eV range;
these Q-balls may survive untill present.Comment: 9 pages, 1 figur
QuickSpec: Guessing Formal Specifications using Testing
We present QuickSpec, a tool that automatically generates algebraic specifications for sets of pure functions. The tool is based on testing, rather than static analysis or theorem proving. The main challenge QuickSpec faces is to keep the number of generated equations to a minimum while maintaining completeness. We demonstrate how QuickSpec can improve one’s understanding of a program module by exploring the laws that are generated using two case studies: a heap library for Haskell and a fixed-point arithmetic library for Erlang
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