15,688 research outputs found
Combinatorial realizations of crystals via torus actions on quiver varieties
Consider Kashiwara's crystal associated to a highest weight representation of
a symmetric Kac-Moody algebra. There is a geometric realization of this object
using Nakajima's quiver varieties, but in many particular cases it can also be
realized by elementary combinatorial methods. Here we propose a framework for
extracting combinatorial realizations from the geometric picture: We construct
certain torus actions on the quiver varieties and use Morse theory to index the
irreducible components by connected components of the subvariety of torus fixed
points. We then discuss the case of affine sl(n). There the fixed point
components are just points, and are naturally indexed by multi-partitions.
There is some choice in our construction, leading to a family of combinatorial
models for each highest weight crystal. Applying this construction to the
crystal of the fundamental representation recovers a family of combinatorial
realizations recently constructed by Fayers. This gives a more conceptual proof
of Fayers' result as well as a generalization to higher level. We also discuss
a relationship with Nakajima's monomial crystal.Comment: 23 pages, v2: added Section 8 on monomial crystals and some
references; v3: many small correction
Omniscopes: Large Area Telescope Arrays with only N log N Computational Cost
We show that the class of antenna layouts for telescope arrays allowing cheap
analysis hardware (with correlator cost scaling as N log N rather than N^2 with
the number of antennas N) is encouragingly large, including not only previously
discussed rectangular grids but also arbitrary hierarchies of such grids, with
arbitrary rotations and shears at each level. We show that all correlations for
such a 2D array with an n-level hierarchy can be efficiently computed via a
Fast Fourier Transform in not 2 but 2n dimensions. This can allow major
correlator cost reductions for science applications requiring exquisite
sensitivity at widely separated angular scales, for example 21cm tomography
(where short baselines are needed to probe the cosmological signal and long
baselines are needed for point source removal), helping enable future 21cm
experiments with thousands or millions of cheap dipole-like antennas. Such
hierarchical grids combine the angular resolution advantage of traditional
array layouts with the cost advantage of a rectangular Fast Fourier Transform
Telescope. We also describe an algorithm for how a subclass of hierarchical
arrays can efficiently use rotation synthesis to produce global sky maps with
minimal noise and a well-characterized synthesized beam.Comment: Replaced to match accepted PRD version. 10 pages, 9 fig
Home equity withdrawal in retirement
The authors study empirically and theoretically the patterns of home equity withdrawal among retirees, using a model in which retirees are able to own or rent a home, save, and borrow against home equity, in the face of idiosyncratic risks concerning mortality, health, medical expenditures, and household size and observed house price changes. The estimated model is found to successfully replicate the patterns of homeownership and the saving/borrowing decisions of retirees. They use the estimated model for several counterfactual experiments. There are three main findings. First, the model predicts that a house price boom suppresses homeownership and increases borrowing, while a decline in house prices has the opposite effect. Second, the costs of home equity borrowing restrict the borrowing of retirees, and thus a reduction of such costs (e.g., lower costs of reverse mortgage loans) might significantly raise home equity borrowing. Third, there are two implications for the retirement saving puzzle. Although the cost of borrowing against equity in the house affects the borrowing of retirees, it does not affect total asset holding, implying that equity borrowing costs do not seem to offer a quantitatively significant contribution to resolving the retirement saving puzzle. On the other hand, the magnitude of the retirement saving puzzle might be exaggerated, because a sizable part of "retirement saving" is due to house price appreciation.Home equity loans ; Retirement
Excitation spectrum of Bilayer Quantum Hall Systems
Excitation spectra in bilayer quantum Hall systems at total Landau-level
filling are studied by the Hartree-Fock-Bogoliubov approximation. The
systems have the spin degrees of freedom in addition to the layer degrees of
freedom described in terms of pseudospin. On the excitation spectra from
spin-unpolarized and pseudospin-polarized ground state, this approximation
fully preserves the spin rotational symmetry and thus can give not only
spin-triplet but also spin-singlet excitations systematically. It is also found
that the ground-state properties are well described by this approximation.Comment: 5 pages, 3 figures; conference: EP2DS-1
Are U.S. and Seventh District business cycles alike?
This article explains the recent high levels of residential investment and rates of homeownership.Business cycles
Status of FNAL SciBooNE experiment
SciBooNE is a new experiment at FNAL which will make precision
neutrino-nucleus cross section measurements in the one GeV region. These
measurements are essential for the future neutrino oscillation experiments. We
started data taking in the antineutrino mode on June 8, 2007, and collected
5.19 \times 10^{19} protons on target (POT) before the accelerator shutdown in
August. The first data from SciBooNE are reported in this article.Comment: 3 pages, 3 figures. Proceedings of the 10th International Conference
on Topics in Astroparticle and Underground Physics (TAUP) 2007, Sendai,
Japan, September 11-15, 200
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