3,001 research outputs found
Optimal Principal Component Analysis in Distributed and Streaming Models
We study the Principal Component Analysis (PCA) problem in the distributed
and streaming models of computation. Given a matrix a
rank parameter , and an accuracy parameter , we
want to output an orthonormal matrix for which where is the best rank- approximation to .
This paper provides improved algorithms for distributed PCA and streaming
PCA.Comment: STOC2016 full versio
The failure of the Tacoma Narrows Bridge
The Board of Engineers appointed by you to
report on the Failure of the Tacoma Narrows Bridge have
made a complete investigation of the design, the behaviour
after completion and the failure of the structure. Our
report covering this investigation follows
Cosmogenic \u3csup\u3e26\u3c/sup\u3eAl/\u3csup\u3e10\u3c/sup\u3eBe surface production ratio in Greenland
The assumed value for the cosmogenic 26Al/10Be surface production rate ratio in quartz is an important parameter for studies investigating the burial or subaerial erosion of long-lived surfaces and sediments. Recent models and data suggest that the production ratio is spatially variable and may be greater than originally thought. Here we present measured 26Al/10Be ratios for 24 continuously exposed bedrock and boulder surfaces spanning ~61–77°N in Greenland. Empirical measurements, such as ours, include nuclides produced predominately by neutron-induced spallation with percent-level contributions by muon interactions. The slope of a York regression line fit to our data is 7.3 ± 0.3 (1σ), suggesting that the 26Al/10Be surface production ratio exceeds the commonly used value of 6.75, at least in the Arctic. A higher 26Al/10Be production ratio has implications for multinuclide cosmogenic isotope studies because it results in greater modeled burial durations and erosion rates
CHERI: a research platform deconflating hardware virtualisation and protection
Contemporary CPU architectures conflate virtualization and protection,
imposing virtualization-related performance, programmability,
and debuggability penalties on software requiring finegrained
protection. First observed in micro-kernel research, these
problems are increasingly apparent in recent attempts to mitigate
software vulnerabilities through application compartmentalisation.
Capability Hardware Enhanced RISC Instructions (CHERI) extend
RISC ISAs to support greater software compartmentalisation.
CHERI’s hybrid capability model provides fine-grained compartmentalisation
within address spaces while maintaining software
backward compatibility, which will allow the incremental deployment
of fine-grained compartmentalisation in both our most trusted
and least trustworthy C-language software stacks. We have implemented
a 64-bit MIPS research soft core, BERI, as well as a
capability coprocessor, and begun adapting commodity software
packages (FreeBSD and Chromium) to execute on the platform
Recommended from our members
Seventh international conference on time-resolved vibrational spectroscopy
The International Conference on Time-Resolved Vibrational Spectroscopy (TRVS) is widely recognized as the major international forum for the discussion of advances in this rapidly growing field. The 1995 conference was the seventh in a series that began at Lake Placid, New York, 1982. Santa Fe, New Mexico, was the site of the Seventh International Conference on Time-Resolved Vibrational Spectroscopy, held from June 11 to 16, 1995. TRVS-7 was attended by 157 participants from 16 countries and 85 institutions, and research ranging across the full breadth of the field of time-resolved vibrational spectroscopy was presented. Advances in both experimental capabilities for time-resolved vibrational measurements and in theoretical descriptions of time-resolved vibrational methods continue to occur, and several sessions of the conference were devoted to discussion of these advances and the associated new directions in TRVS. Continuing the interdisciplinary tradition of the TRVS meetings, applications of time-resolved vibrational methods to problems in physics, biology, materials science, and chemistry comprised a large portion of the papers presented at the conference
Determination of the (3x3)-Sn/Ge(111) structure by photoelectron diffraction
At a coverage of about 1/3 monolayer, Sn deposited on Ge(111) below 550 forms
a metastable (sqrt3 x sqrt3)R30 phase. This phase continuously and reversibly
transforms into a (3x3) one, upon cooling below 200 K. The photoemission
spectra of the Sn 4d electrons from the (3x3)-Sn/Ge(111) surface present two
components which are attributed to inequivalent Sn atoms in T4 bonding sites.
This structure has been explored by photoelectron diffraction experiments
performed at the ALOISA beamline of the Elettra storage ring in Trieste
(Italy). The modulation of the intensities of the two Sn components, caused by
the backscattering of the underneath Ge atoms, has been measured as a function
of the emission angle at fixed kinetic energies and viceversa. The bond angle
between Sn and its nearest neighbour atoms in the first Ge layer (Sn-Ge1) has
been measured by taking polar scans along the main symmetry directions and it
was found almost equivalent for the two components. The corresponding bond
lengths are also quite similar, as obtained by studying the dependence on the
photoelectron kinetic energy, while keeping the photon polarization and the
collection direction parallel to the Sn-Ge1 bond orientation (bond emission). A
clear difference between the two bonding sites is observed when studying the
energy dependence at normal emission, where the sensitivity to the Sn height
above the Ge atom in the second layer is enhanced. This vertical distance is
found to be 0.3 Angstroms larger for one Sn atom out of the three contained in
the lattice unit cell. The (3x3)-Sn/Ge(111) is thus characterized by a
structure where the Sn atom and its three nearest neighbour Ge atoms form a
rather rigid unit that presents a strong vertical distortion with respect to
the underneath atom of the second Ge layer.Comment: 10 pages with 9 figures, added reference
The dynamic atmospheres of Mira stars: comparing the CODEX models to PTI time series of TU And
Our comprehension of stellar evolution on the AGB still faces many
difficulties. To improve on this, a quantified understanding of large-amplitude
pulsator atmospheres and interpretation in terms of their fundamental stellar
parameters are essential. We wish to evaluate the effectiveness of the recently
released CODEX dynamical model atmospheres in representing M-type Mira
variables through a confrontation with the time-resolved spectro-photometric
and interferometric PTI data set of TU And. We calibrated the interferometric
K-band time series to high precision. This results in 50 nights of
observations, covering 8 subsequent pulsation cycles. At each phase, the flux
at 2.2m is obtained, along with the spectral shape and visibility points
in 5 channels across the K-band. We compared the data set to the relevant
dynamical, self-excited CODEX models. Both spectrum and visibilities are
consistently reproduced at visual minimum phases. Near maximum, our
observations show that the current models predict a photosphere that is too
compact and hot, and we find that the extended atmosphere lacks H2O opacity.
Since coverage in model parameter space is currently poor, more models are
needed to make firm conclusions on the cause of the discrepancies. We argue
that for TU And, the discrepancy could be lifted by adopting a lower value of
the mixing length parameter combined with an increase in the stellar mass
and/or a decrease in metallicity, but this requires the release of an extended
model grid.Comment: 5 pages, 5 figures, accepted for publication in A&A Letter
- …