8,668 research outputs found
A note on the asymptotics of the number of O-sequences of given length
We look at the number of -sequences of length . Recall that an
-sequence can be defined algebraically as the Hilbert function of a standard
graded -algebra, or combinatorially as the -vector of a multicomplex. The
sequence was first investigated in a recent paper by commutative
algebraists Enkosky and Stone, inspired by Huneke. In this note, we
significantly improve both of their upper and lower bounds, by means of a very
short partition-theoretic argument. In particular, it turns out that, for
suitable positive constants and and all , It remains an open problem to determine an
exact asymptotic estimate for .Comment: Final version to appear in Discrete Mathematics. 2 page
A generalization of a 1998 unimodality conjecture of Reiner and Stanton
An interesting, and still wide open, conjecture of Reiner and Stanton
predicts that certain "strange" symmetric differences of -binomial
coefficients are always nonnegative and unimodal. We extend their conjecture to
a broader, and perhaps more natural, framework, by conjecturing that, for each
, the polynomials
are nonnegative and unimodal for all and
such that (mod 2), with the only exception of
when this is an integer.
Using the KOH theorem, we combinatorially show the case . In fact, we
completely characterize the nonnegativity and unimodality of for
. (This also provides an isolated counterexample to Reiner-Stanton's
conjecture when .) Further, we prove that, for each and , it
suffices to show our conjecture for the largest values of .Comment: Final version. To appear in the Journal of Combinatoric
Time Consistent Policy in Markov Switching Models
In this paper we consider the quadratic optimal control problem with regime shifts and forward-looking agents. This extends the results of Zampolli (2003) who considered models without forward-looking expectations. Two algorithms are presented: The first algorithm computes the solution of a rational expectation model with random parameters or regime shifts. The second algorithm computes the time-consistent policy and the resulting Nash-Stackelberg equilibrium. The formulation of the problem is of general form and allows for model uncertainty and incorporation of policymaker’s judgement. We apply these methods to compute the optimal (non-linear) monetary policy in a small open economy subject to (symmetric or asymmetric) risks of change in some of its key parameters such as inflation inertia, degree of exchange rate pass-through, elasticity of aggregate demand to interest rate, etc.. We normally find that the time-consistent response to risk is more cautious. Furthermore, the optimal response is in some cases non-monotonic as a function of uncertainty. We also simulate the model under assumptions that the policymaker and the private sector hold the same beliefs over the probabilities of the structural change and different beliefs (as well as different assumptions about the knowledge of each other’s reaction function).monetary policy, regime switching, model uncertainty, time consistency
Sub-ohmic two-level system representation of the Kondo effect
It has been recently shown that the particle-hole symmetric Anderson impurity
model can be mapped onto a slave-spin theory without any need of
additional constraints. Here we prove by means of Numerical Renormalization
Group that the slave-spin behaves in this model like a two-level system coupled
to a sub-ohmic dissipative environment. It follows that the symmetry gets
spontaneously broken at zero temperature, which we find can be identified with
the on-set of Kondo coherence, being the Kondo temperature proportional to the
square of the order parameter. Since the model is numerically solvable, the
results are very enlightening on the role of quantum fluctuations beyond mean
field in the context of slave-boson approaches to correlated electron models,
an issue that has been attracting interest since the 80's. Finally, our results
suggest as a by-product that the paramagnetic metal phase of the Hubbard model
at half-filling, in infinite coordination lattices and at zero temperature, as
described for instance by Dynamical Mean Field Theory, corresponds to a
slave-spin theory with a spontaneous breakdown of a local gauge symmetry.Comment: 4 pages, 5 figure
Theory of the Metal-Paramagnetic Mott-Jahn-Teller Insulator Transition in A_4C_{60}
We study the unconventional insulating state in A_4C_{60} with a variety of
approaches, including density functional calculations and dynamical mean-field
theory. While the former predicts a metallic state, in disagreement with
experiment, the latter yields a (paramagnetic) Mott-Jahn-Teller insulator. In
that state, conduction between molecules is blocked by on-site Coulomb
repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and
important excitations (such as optical and spin gap) should be essentially
intra-molecular. Experimental gaps of 0.5 eV and 0.1 eV respectively compare
well with molecular ion values, in agreement with this picture.Comment: 4 pages, 2 postscript figure
Quantification and scaling of multipartite entanglement in continuous variable systems
We present a theoretical method to determine the multipartite entanglement
between different partitions of multimode, fully or partially symmetric
Gaussian states of continuous variable systems. For such states, we determine
the exact expression of the logarithmic negativity and show that it coincides
with that of equivalent two--mode Gaussian states. Exploiting this reduction,
we demonstrate the scaling of the multipartite entanglement with the number of
modes and its reliable experimental estimate by direct measurements of the
global and local purities.Comment: 4 pages, 2 figures; to be published in Phys. Rev. Let
Gaia Data Release 1. Cross-match with external catalogues - Algorithm and results
Although the Gaia catalogue on its own will be a very powerful tool, it is
the combination of this highly accurate archive with other archives that will
truly open up amazing possibilities for astronomical research. The advanced
interoperation of archives is based on cross-matching, leaving the user with
the feeling of working with one single data archive. The data retrieval should
work not only across data archives, but also across wavelength domains. The
first step for seamless data access is the computation of the cross-match
between Gaia and external surveys. The matching of astronomical catalogues is a
complex and challenging problem both scientifically and technologically
(especially when matching large surveys like Gaia). We describe the cross-match
algorithm used to pre-compute the match of Gaia Data Release 1 (DR1) with a
selected list of large publicly available optical and IR surveys. The overall
principles of the adopted cross-match algorithm are outlined. Details are given
on the developed algorithm, including the methods used to account for position
errors, proper motions, and environment; to define the neighbours; and to
define the figure of merit used to select the most probable counterpart.
Statistics on the results are also given. The results of the cross-match are
part of the official Gaia DR1 catalogue.Comment: 18 pages, 8 figures. Accepted for publication by A&
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