23,362 research outputs found
Isospectral Graph Reductions and Improved Estimates of Matrices' Spectra
Via the process of isospectral graph reduction the adjacency matrix of a
graph can be reduced to a smaller matrix while its spectrum is preserved up to
some known set. It is then possible to estimate the spectrum of the original
matrix by considering Gershgorin-type estimates associated with the reduced
matrix. The main result of this paper is that eigenvalue estimates associated
with Gershgorin, Brauer, Brualdi, and Varga improve as the matrix size is
reduced. Moreover, given that such estimates improve with each successive
reduction, it is also possible to estimate the eigenvalues of a matrix with
increasing accuracy by repeated use of this process.Comment: 32 page
Faint star counts in the near-infrared
We discuss near-infrared star counts at the Galactic pole with a view to
guiding the NGST and ground-based NIR cameras. Star counts from deep K-band
images from the CFHT are presented, and compared with results from the 2MASS
survey and some Galaxy models. With appropriate corrections for detector
artifacts and galaxies, the data agree with the models down to K~18, but
indicate a larger population of fainter red stars. There is also a significant
population of compact galaxies that extend to the observational faint limit of
K=20.5. Recent Galaxy models agree well down to K19, but diverge at
fainter magnitudes.Comment: 14 pages and 4 diagrams; to appear in PAS
Retort braze bonding of borsic/aluminum composite sheet to titanium
Braze bonding studies between Borsic/aluminum composite and titanium sheet were conducted to establish acceptable brazing techniques and to assess potential joint efficiencies. Excellent braze joints were produced which exhibited joint strengths exceeding 117 MPa (17,000 psi) and which retained up to 2/3 of this strength at 589 K (600 F). Noticeable composite strength degradation resulting from the required high temperature braze cycle was found to be a problem
Gain control from beyond the classical receptive field in primate primary visual cortex
Gain control is a salient feature of information processing throughout the visual system. Heeger (1991, 1992) described a mechanism that could underpin gain control in primary visual cortex (VI). According to this model, a neuron's response is normalized by dividing its output by the sum of a population of neurons, which are selective for orientations covering a broad range. Gain control in this scheme is manifested as a change in the semisaturation constant (contrast gain) of a VI neuron. Here we examine how flanking and annular gratings of the same or orthogonal orientation to that preferred by a neuron presented beyond the receptive field modulate gain in V1 neurons in anesthetized marmosets (Callithrix jacchus). To characterize how gain was modulated by surround stimuli, the Michaelis-Menten equation was fitted to response versus contrast functions obtained under each stimulus condition. The modulation of gain by surround stimuli was modelled best as a divisive reduction in response gain. Response gain varied with the orientation of surround stimuli, but was reduced most when the orientation of a large annular grating beyond the classical receptive field matched the preferred orientation of neurons. The strength of surround suppression did not vary significantly with retinal eccentricity or laminar distribution. In the mannoset, as in macaques (Angelucci et al., 2002a,b), gain control over the sort of distances reported here (up to 10 deg) may be mediated by feedback from extrastriate areas
Presidential Position-Taking, Presidential Success, and Interest Group Activity
This is the author acepted manuscript. The final version is available from Taylor & Francis via the DOI in this recordDo lobby groups help the American president achieve policy objectives? Existing research seldom evaluates interest groups and the president in conjunction, and as a result we have little systematic knowledge about how groups respond to presidential actions or whether they assist in realizing the president’s policy agenda. Building on existing data obtained through interviews with 776 lobbyists, combined with variables we generate describing issue salience, congressional attention, the political context, and policy adoption, we show that interest groups adjusted their lobbying activity to better reflect the president’s voiced preferences. Despite this strategy, we find that lobby groups had no significant marginal effect on policy adoption when controlling for the overwhelming influence of the president. The strong association between policy adoption and position-taking by the president withstands the inclusion of five alternative variables found in previous studies to condition the influence of the president over policy adoption
Microwave-Induced Dephasing in One-Dimensional Metal Wires
We report on the effect of monochromatic microwave (MW) radiation on the weak
localization corrections to the conductivity of quasi-one-dimensional (1D)
silver wires. Due to the improved electron cooling in the wires, the MW-induced
dephasing was observed without a concomitant overheating of electrons over wide
ranges of the MW power and frequency . The observed dependences of
the conductivity and MW-induced dephasing rate on and are in
agreement with the theory by Altshuler, Aronov, and Khmelnitsky \cite{Alt81}.
Our results suggest that in the low-temperature experiments with 1D wires,
saturation of the temperature dependence of the dephasing time can be caused by
an MW electromagnetic noise with a sub-pW power.Comment: 4 pages with 4 figures, paper revised, accepted by Phys Rev Let
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