41,771 research outputs found
Testing the Accuracy and Stability of Spectral Methods in Numerical Relativity
The accuracy and stability of the Caltech-Cornell pseudospectral code is
evaluated using the KST representation of the Einstein evolution equations. The
basic "Mexico City Tests" widely adopted by the numerical relativity community
are adapted here for codes based on spectral methods. Exponential convergence
of the spectral code is established, apparently limited only by numerical
roundoff error. A general expression for the growth of errors due to finite
machine precision is derived, and it is shown that this limit is achieved here
for the linear plane-wave test. All of these tests are found to be stable,
except for simulations of high amplitude gauge waves with nontrivial shift.Comment: Final version, as published in Phys. Rev. D; 13 pages, 16 figure
Consistent Second-Order Conic Integer Programming for Learning Bayesian Networks
Bayesian Networks (BNs) represent conditional probability relations among a
set of random variables (nodes) in the form of a directed acyclic graph (DAG),
and have found diverse applications in knowledge discovery. We study the
problem of learning the sparse DAG structure of a BN from continuous
observational data. The central problem can be modeled as a mixed-integer
program with an objective function composed of a convex quadratic loss function
and a regularization penalty subject to linear constraints. The optimal
solution to this mathematical program is known to have desirable statistical
properties under certain conditions. However, the state-of-the-art optimization
solvers are not able to obtain provably optimal solutions to the existing
mathematical formulations for medium-size problems within reasonable
computational times. To address this difficulty, we tackle the problem from
both computational and statistical perspectives. On the one hand, we propose a
concrete early stopping criterion to terminate the branch-and-bound process in
order to obtain a near-optimal solution to the mixed-integer program, and
establish the consistency of this approximate solution. On the other hand, we
improve the existing formulations by replacing the linear "big-" constraints
that represent the relationship between the continuous and binary indicator
variables with second-order conic constraints. Our numerical results
demonstrate the effectiveness of the proposed approaches
Quantization of anomaly coefficients in 6D supergravity
We obtain new constraints on the anomaly coefficients of 6D
supergravity theories using local and global anomaly
cancellation conditions. We show how these constraints can be strengthened if
we assume that the theory is well-defined on any spin space-time with an
arbitrary gauge bundle. We distinguish the constraints depending on the gauge
algebra only from those depending on the global structure of the gauge group.
Our main constraint states that the coefficients of the anomaly polynomial for
the gauge group should be an element of where is the unimodular string charge lattice. We show
that the constraints in their strongest form are realized in F-theory
compactifications. In the process, we identify the cocharacter lattice, which
determines the global structure of the gauge group, within the homology lattice
of the compactification manifold.Comment: 42 pages. v3: Some clarifications, typos correcte
Integer-Forcing Source Coding
Integer-Forcing (IF) is a new framework, based on compute-and-forward, for
decoding multiple integer linear combinations from the output of a Gaussian
multiple-input multiple-output channel. This work applies the IF approach to
arrive at a new low-complexity scheme, IF source coding, for distributed lossy
compression of correlated Gaussian sources under a minimum mean squared error
distortion measure. All encoders use the same nested lattice codebook. Each
encoder quantizes its observation using the fine lattice as a quantizer and
reduces the result modulo the coarse lattice, which plays the role of binning.
Rather than directly recovering the individual quantized signals, the decoder
first recovers a full-rank set of judiciously chosen integer linear
combinations of the quantized signals, and then inverts it. In general, the
linear combinations have smaller average powers than the original signals. This
allows to increase the density of the coarse lattice, which in turn translates
to smaller compression rates. We also propose and analyze a one-shot version of
IF source coding, that is simple enough to potentially lead to a new design
principle for analog-to-digital converters that can exploit spatial
correlations between the sampled signals.Comment: Submitted to IEEE Transactions on Information Theor
Half-integer Higher Spin Fields in (A)dS from Spinning Particle Models
We make use of O(2r+1) spinning particle models to construct linearized
higher-spin curvatures in (A)dS spaces for fields of arbitrary half-integer
spin propagating in a space of arbitrary (even) dimension: the field
potentials, whose curvatures are computed with the present models, are
spinor-tensors of mixed symmetry corresponding to Young tableaux with D/2 - 1
rows and r columns, thus reducing to totally symmetric spinor-tensors in four
dimensions. The paper generalizes similar results obtained in the context of
integer spins in (A)dS.Comment: 1+18 pages; minor changes in the notation, references updated.
Published versio
A One-Parameter Family of Hamiltonian Structures for the KP Hierarchy and a Continuous Deformation of the Nonlinear \W_{\rm KP} Algebra
The KP hierarchy is hamiltonian relative to a one-parameter family of Poisson
structures obtained from a generalized Adler map in the space of formal
pseudodifferential symbols with noninteger powers. The resulting \W-algebra
is a one-parameter deformation of \W_{\rm KP} admitting a central extension
for generic values of the parameter, reducing naturally to \W_n for special
values of the parameter, and contracting to the centrally extended
\W_{1+\infty}, \W_\infty and further truncations. In the classical limit,
all algebras in the one-parameter family are equivalent and isomorphic to
\w_{\rm KP}. The reduction induced by setting the spin-one field to zero
yields a one-parameter deformation of \widehat{\W}_\infty which contracts to
a new nonlinear algebra of the \W_\infty-type.Comment: 31 pages, compressed uuencoded .dvi file, BONN-HE-92/20, US-FT-7/92,
KUL-TF-92/20. [version just replaced was truncated by some mailer
Fluxes in F-theory Compactifications on Genus-One Fibrations
We initiate the construction of gauge fluxes in F-theory compactifications on
genus-one fibrations which only have a multi-section as opposed to a section.
F-theory on such spaces gives rise to discrete gauge symmetries in the
effective action. We generalize the transversality conditions on gauge fluxes
known for elliptic fibrations by taking into account the properties of the
available multi-section. We test these general conditions by constructing all
vertical gauge fluxes in a bisection model with gauge group SU(5) x Z2. The
non-abelian anomalies are shown to vanish. These flux solutions are dynamically
related to fluxes on a fibration with gauge group SU(5) x U(1) by a conifold
transition. Considerations of flux quantization reveal an arithmetic constraint
on certain intersection numbers on the base which must necessarily be satisfied
in a smooth geometry. Combined with the proposed transversality conditions on
the fluxes these conditions are shown to imply cancellation of the discrete Z2
gauge anomalies as required by general consistency considerations.Comment: 30 pages; v2: typos correcte
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