331 research outputs found
The Shareholder Wealth Effects of Director Departure Announcements
While executive directors are responsible for superior performance, their behaviour may not always be aligned with shareholder interests. Non-executive directors provide one method of monitoring and controlling these and other agency costs. An event study focusing on director departure provides some insight into the economic importance of directors to shareholders. Initial results highlight both the importance of non-executive directors relative to other directors and the possibility of performance and size impacts. Multivariate tests suggest that non-executive director departures, especially when combined with resignation, explain the cross-sectional variation in share returns associated with director departure even after controlling for performance and size
Fast Searching in Packed Strings
Given strings and the (exact) string matching problem is to find all
positions of substrings in matching . The classical Knuth-Morris-Pratt
algorithm [SIAM J. Comput., 1977] solves the string matching problem in linear
time which is optimal if we can only read one character at the time. However,
most strings are stored in a computer in a packed representation with several
characters in a single word, giving us the opportunity to read multiple
characters simultaneously. In this paper we study the worst-case complexity of
string matching on strings given in packed representation. Let be
the lengths and , respectively, and let denote the size of the
alphabet. On a standard unit-cost word-RAM with logarithmic word size we
present an algorithm using time O\left(\frac{n}{\log_\sigma n} + m +
\occ\right). Here \occ is the number of occurrences of in . For this improves the bound of the Knuth-Morris-Pratt algorithm.
Furthermore, if our algorithm is optimal since any
algorithm must spend at least \Omega(\frac{(n+m)\log
\sigma}{\log n} + \occ) = \Omega(\frac{n}{\log_\sigma n} + \occ) time to
read the input and report all occurrences. The result is obtained by a novel
automaton construction based on the Knuth-Morris-Pratt algorithm combined with
a new compact representation of subautomata allowing an optimal
tabulation-based simulation.Comment: To appear in Journal of Discrete Algorithms. Special Issue on CPM
200
Curved Tails in Polymerization-Based Bacterial Motility
The curved actin ``comet-tail'' of the bacterium Listeria monocytogenes is a
visually striking signature of actin polymerization-based motility. Similar
actin tails are associated with Shigella flexneri, spotted-fever Rickettsiae,
the Vaccinia virus, and vesicles and microspheres in related in vitro systems.
We show that the torque required to produce the curvature in the tail can arise
from randomly placed actin filaments pushing the bacterium or particle. We find
that the curvature magnitude determines the number of actively pushing
filaments, independent of viscosity and of the molecular details of force
generation. The variation of the curvature with time can be used to infer the
dynamics of actin filaments at the bacterial surface.Comment: 8 pages, 2 figures, Latex2
The road to deterministic matrices with the restricted isometry property
The restricted isometry property (RIP) is a well-known matrix condition that
provides state-of-the-art reconstruction guarantees for compressed sensing.
While random matrices are known to satisfy this property with high probability,
deterministic constructions have found less success. In this paper, we consider
various techniques for demonstrating RIP deterministically, some popular and
some novel, and we evaluate their performance. In evaluating some techniques,
we apply random matrix theory and inadvertently find a simple alternative proof
that certain random matrices are RIP. Later, we propose a particular class of
matrices as candidates for being RIP, namely, equiangular tight frames (ETFs).
Using the known correspondence between real ETFs and strongly regular graphs,
we investigate certain combinatorial implications of a real ETF being RIP.
Specifically, we give probabilistic intuition for a new bound on the clique
number of Paley graphs of prime order, and we conjecture that the corresponding
ETFs are RIP in a manner similar to random matrices.Comment: 24 page
Coronin Promotes the Rapid Assembly and Cross-linking of Actin Filaments and May Link the Actin and Microtubule Cytoskeletons in Yeast
Coronin is a highly conserved actin-associated protein that until now has had unknown biochemical activities. Using microtubule affinity chromatography, we coisolated actin and a homologue of coronin, Crn1p, from Saccharomyces cerevisiae cell extracts. Crn1p is an abundant component of the cortical actin cytoskeleton and binds to F-actin with high affinity (Kd 6 × 10−9 M). Crn1p promotes the rapid barbed-end assembly of actin filaments and cross-links filaments into bundles and more complex networks, but does not stabilize them. Genetic analyses with a crn1Δ deletion mutation also are consistent with Crn1p regulating filament assembly rather than stability. Filament cross-linking depends on the coiled coil domain of Crn1p, suggesting a requirement for Crn1p dimerization. Assembly-promoting activity is independent of cross-linking and could be due to nucleation and/or accelerated polymerization. Crn1p also binds to microtubules in vitro, and microtubule binding is enhanced by the presence of actin filaments. Microtubule binding is mediated by a region of Crn1p that contains sequences (not found in other coronins) homologous to the microtubule binding region of MAP1B. These activities, considered with microtubule defects observed in crn1Δ cells and in cells overexpressing Crn1p, suggest that Crn1p may provide a functional link between the actin and microtubule cytoskeletons in yeast
A Binary Lensing Event Toward the LMC: Observations and Dark Matter Implications
The MACHO collaboration has recently analyzed 2.1 years of photometric data
for about 8.5 million stars in the Large Magellanic Cloud (LMC). This analysis
has revealed 8 candidate microlensing events and a total microlensing optical
depth of . This significantly
exceeds the number of events (1.1) and the microlensing optical depth predicted
from known stellar populations: , but it is
consistent with models in which about half of the standard dark halo mass is
composed of Machos of mass \sim 0.5 \msun. One of these 8 events appears to
be a binary lensing event with a caustic crossing that is partially resolved
which allows us to estimate the distance to the lenses. If the source star is
not a short period binary star, then we show that the lens system is very
likely to reside in the LMC. However, if we assume that the optical depth for
LMC-LMC lensing is large enough to account for our entire lensing signal, then
the binary event does not appear to be consistent with lensing of a single LMC
source star by a binary residing in the LMC. Thus, while the binary lens may
indeed reside in the LMC, there is no indication that most of the lenses reside
in the LMC.Comment: 5 pages, 3 postscript figures included; To appear in the Proceedings
of the Dark Matter '96 Conference held in Santa Monica, CA, Feb., 199
The MACHO Project 2nd Year LMC Microlensing Results and Dark Matter Implications
The MACHO Project is searching for galactic dark matter in the form of
massive compact halo objects (Machos). Millions of stars in the Large
Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge are
photometrically monitored in an attempt to detect rare gravitational
microlensing events caused by otherwise invisible Machos. Analysis of two years
of photometry on 8.5 million stars in the LMC reveals 8 candidate microlensing
events, far more than the event expected from lensing by low-mass stars
in known galactic populations. From these eight events we estimate the optical
depth towards the LMC from events with 2 < \that < 200 days to be
\tau_2^{200} \approx 2.9 ^{+1.4}_{-0.9} \ten{-7}. This exceeds the optical
depth of 0.5\ten{-7} expected from known stars and is to be compared with an
optical depth of 4.7\ten{-7} predicted for a ``standard'' halo composed
entirely of Machos. The total mass in this lensing population is \approx
2^{+1.2}_{-0.7} \ten{11} \msun (within 50 kpc from the Galactic center). Event
timescales yield a most probable Macho mass of 0.5^{+0.3}_{-0.2}\msun,
although this value is quite model dependent.Comment: 10 pages, 6 epsf figures and style file included, 451k, also at
http://wwwmacho.mcmaster.ca/Pubs/Pubs.html; To appear in the Proceedings of
"Sources and Detection of Dark Matter in the Universe", Santa Monica, CA,
Feb., 199
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