16,222 research outputs found
Diquark interaction and gaps for color superconductivity
Using flow equations, we derive an effective quark-quark interaction and
obtain the coupled set of gap equations for the condensates of the CFL phase of
massless dense QCD. The formalism developed here enables one to
consider more general case of nonzero -quark mass.Comment: 3 pages, Lattice2002(nonzerot), talk presented at 20th International
Symposium on Lattice Field Theory (LATTICE2002), Boston, Massachusetts, 24-29
June 200
Answer Sets for Logic Programs with Arbitrary Abstract Constraint Atoms
In this paper, we present two alternative approaches to defining answer sets
for logic programs with arbitrary types of abstract constraint atoms (c-atoms).
These approaches generalize the fixpoint-based and the level mapping based
answer set semantics of normal logic programs to the case of logic programs
with arbitrary types of c-atoms. The results are four different answer set
definitions which are equivalent when applied to normal logic programs. The
standard fixpoint-based semantics of logic programs is generalized in two
directions, called answer set by reduct and answer set by complement. These
definitions, which differ from each other in the treatment of
negation-as-failure (naf) atoms, make use of an immediate consequence operator
to perform answer set checking, whose definition relies on the notion of
conditional satisfaction of c-atoms w.r.t. a pair of interpretations. The other
two definitions, called strongly and weakly well-supported models, are
generalizations of the notion of well-supported models of normal logic programs
to the case of programs with c-atoms. As for the case of fixpoint-based
semantics, the difference between these two definitions is rooted in the
treatment of naf atoms. We prove that answer sets by reduct (resp. by
complement) are equivalent to weakly (resp. strongly) well-supported models of
a program, thus generalizing the theorem on the correspondence between stable
models and well-supported models of a normal logic program to the class of
programs with c-atoms. We show that the newly defined semantics coincide with
previously introduced semantics for logic programs with monotone c-atoms, and
they extend the original answer set semantics of normal logic programs. We also
study some properties of answer sets of programs with c-atoms, and relate our
definitions to several semantics for logic programs with aggregates presented
in the literature
The mean-square dichotomy spectrum and a bifurcation to a mean-square attractor
The dichotomy spectrum is introduced for linear mean-square random dynamical
systems, and it is shown that for finite-dimensional mean-field stochastic
differential equations, the dichotomy spectrum consists of finitely many
compact intervals. It is then demonstrated that a change in the sign of the
dichotomy spectrum is associated with a bifurcation from a trivial to a
non-trivial mean-square random attractor
Drastic Reduction of Shot Noise in Semiconductor Superlattices
We have found experimentally that the shot noise of the tunneling current
through an undoped semiconductor superlattice is reduced with respect to the
Poissonian noise value , and that the noise approaches 1/3 of that value
in superlattices whose quantum wells are strongly coupled. On the other hand,
when the coupling is weak or when a strong electric field is applied to the
superlattice the noise becomes Poissonian. Although our results are
qualitatively consistent with existing theories for one-dimensional mulitple
barriers, the theories cannot account for the dependence of the noise on
superlattice parameters that we have observed.Comment: 4 Pages, 3Figure
Kinetics of the photosubstitution of cis-bis(benzonitrile)dichloroplatinum(II) in chloroform
Under 254 nm irradiation cis-[Pt(C6H5CN)2Cl2] is converted to H2PtCl6. Absorption of light by both the metal complex and the solvent contribute to the first step of this process, suggested to form HPt(C6H5CN) Cl3. A linear dependence of the reaction rate on light intensity appears to rule out chlorination by trichloromethyl radicals. However, at higher light intensities a higher order dependence on intensity develops, and under 313 nm irradiation is dominant, and a reaction between trichloromethyl radical and the excited state complex is proposed to account for this
On finite-density QCD at large Nc
Deryagin, Grigoriev, and Rubakov (DGR) have shown that in finite-density QCD
at infinite Nc the Fermi surface is unstable with respect to the formation of
chiral waves with wavenumber twice the Fermi momentum, while the BCS
instability is suppressed. We show here that at large, but finite Nc, the DGR
instability only occurs in a finite window of chemical potentials from above
Lambda_QCD to mu_critical = exp(gamma ln^2 Nc + O(ln Nc ln ln Nc))Lambda_QCD,
where gamma = 0.02173. Our analysis shows that, at least in the perturbative
regime, the instability occurs only at extremely large Nc, Nc > 1000 Nf, where
Nf is the number of flavors. We conclude that the DGR instability is not likely
to occur in QCD with three colors, where the ground state is expected to be a
color superconductor. We speculate on possible structure of the ground state of
finite-density QCD with very large Nc.Comment: 13 pages, 5 figures, 3 figures drawn using PicTe
A kinetic study of the photolysis of tris(2,4-pentanedionato)cobalt(III) and bis(2,4-pentanedionato)cobalt(II) in chloroform
Under 254nm irradiation in chloroform, Co(acac)3 (Hacac = 2,4-pentanedione) is converted to Co(acac)2 and then to CoCl2. The metal complex is the primary photoactive species in the photoreduction of Co(acac)3, but the photosubstitution of Co(acac)2 appears to occur primarily through absorption of light by the solvent, followed by a chain reaction in which chlorine atoms displace pentanedionyl radicals. The photosubstitution rate law is complex, and the apparent quantum yield (based on total light absorbed) varies with incident light intensity and Co(acac)2 concentration, reaching values as high as 16 under the conditions of this study. Referred only to the light absorbed by CHCL3, the highest quantum yield measured was 150. An observed partial inverse dependence of the photosubstitution rate on the initial concentration of Co(acac)2 is explained in terms of a mechanism in which the pentanedione product competes with Co(acac)2 for an intermediate
An Analysis of the Search Spaces for Generate and Validate Patch Generation Systems
We present the first systematic analysis of the characteristics of patch
search spaces for automatic patch generation systems. We analyze the search
spaces of two current state-of-the-art systems, SPR and Prophet, with 16
different search space configurations. Our results are derived from an analysis
of 1104 different search spaces and 768 patch generation executions. Together
these experiments consumed over 9000 hours of CPU time on Amazon EC2.
The analysis shows that 1) correct patches are sparse in the search spaces
(typically at most one correct patch per search space per defect), 2) incorrect
patches that nevertheless pass all of the test cases in the validation test
suite are typically orders of magnitude more abundant, and 3) leveraging
information other than the test suite is therefore critical for enabling the
system to successfully isolate correct patches.
We also characterize a key tradeoff in the structure of the search spaces.
Larger and richer search spaces that contain correct patches for more defects
can actually cause systems to find fewer, not more, correct patches. We
identify two reasons for this phenomenon: 1) increased validation times because
of the presence of more candidate patches and 2) more incorrect patches that
pass the test suite and block the discovery of correct patches. These
fundamental properties, which are all characterized for the first time in this
paper, help explain why past systems often fail to generate correct patches and
help identify challenges, opportunities, and productive future directions for
the field
Logic Programming for Finding Models in the Logics of Knowledge and its Applications: A Case Study
The logics of knowledge are modal logics that have been shown to be effective
in representing and reasoning about knowledge in multi-agent domains.
Relatively few computational frameworks for dealing with computation of models
and useful transformations in logics of knowledge (e.g., to support multi-agent
planning with knowledge actions and degrees of visibility) have been proposed.
This paper explores the use of logic programming (LP) to encode interesting
forms of logics of knowledge and compute Kripke models. The LP modeling is
expanded with useful operators on Kripke structures, to support multi-agent
planning in the presence of both world-altering and knowledge actions. This
results in the first ever implementation of a planner for this type of complex
multi-agent domains.Comment: 16 pages, 1 figure, International Conference on Logic Programming
201
Traversable wormhole in the deformed Ho\v{r}ava-Lifshitz gravity
Asymptotically flat wormhole solutions are found in the deformed
Ho\v{r}ava-Lifshitz gravity. It turns out that higher curvature terms can not
play the role of exotic matters which are crucial to form a traversable
wormhole, and external exotic sources are still needed. In particular, the
exotic matter behaves like phantom energy if Kehagias-Sfetsos vacuum is
considered outside the wormhole. Interestingly, the spherically symmetric
setting makes the matter and the higher curvature contribution satisfy
four-dimensional conservation of energy in the covariant form.Comment: 13 pages, 2 figures, version published in Phys. Rev.
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